How 2 Power Ur House With The Sun
Powering your house from the sun.
In 2016 we sold a lot of mature timber and came into a lot of money in one year. We needed a
tax write off and so looked into solar for our house. Presently we are running the house and two
electric cars and getting a check every year in February for excess production. When we began
this journey 4 years ago, I didn’t know what I know now. We made a few missteps along the
way. It is my hope that you will find this informative and not make the same mistakes we did.
By now, you have seen “too good to be true” and “no money down” solar systems for sale on
the internet. Keep in mind that nothing is free and if you wouldn’t buy a car from a buy here pay
here place, you shouldn’t buy “no money down” solar. A chapter will be devoted to this subject.
Solar has become quite affordable lately and many people don’t even know where to begin. I
hope to answer a lot of questions and prime you to ask the right questions when it is time for
you to pull the trigger.
For the sake of this article, when I refer to a solar system, I’m referring to a solar PV
(photovoltaic) system. There are thermal solar panels that heat water, but my primary interest is
in electrical panels.
Solar PV systems fall into two categories…. Grid Tie and Off Grid.
Grid Tie systems are the cheapest and most commonly sold. With grid tie system, you treat the
grid like a battery or a bank account. During the day your electric meter will run backwards
pushing power into the grid just like you put money into a savings account for a rainy day. At
night, your meter will once again run forwards eating away at that excess power you have in
savings. Your meter will run backwards more in the change over seasons like fall and spring,
and you will tend to draw more power from the grid in the heat of summer or cold of winter.
First and foremost, a Grid Tie system does not make power during a power outage. As a matter
of safety, a Grid Tie solar system will not make power until the grid has been up for 5 minutes.
This is to prevent back feeding power and potentially killing a lineman.
Off-Grid systems have the ability to continue operation when the grid is down or when you
choose to not have electric service. Ideally, they will use as much of your home grown solar
power as possible before drawing from the grid. If you so choose, an off-grid capable system
can also back feed the grid just like the grid tie system and earn you credits with your local
Off Grid systems by definition must use batteries. Batteries until lately have tended to be lead
acid but Lithium batteries are very affordable now. We will discuss batteries more in depth later
in this article.
So let’s get cracking!
Sizing your Solar System.
The first thing you need to know is how much power you will need to make. Since electricity is
produced and sold by kWh, this is what we need to determine. Look on your electric bill and
find the kWh used for the month. Totalize this for one year and divide by 365. For example, you
find that your total electric usage for the year is 18,250kWh. 18250/365 = 50kw per day. So
your goal is to make 50kw a day on average for a year.
Allow me to pause here for a moment. It is very important to know how much power you are
using. Do not guess. It is equally important for you to optimize your house before you do your
solar calculations. The easiest gain can be had by switching your lights to LED. LED lights are
10x more efficient than incandescent, and 2.5x more efficient than fluorescent lamps. In my
installation in 2016, we failed to take into account the lighting in the house and a few 4 tube
fluorescent fixtures in the basement that stayed on 24/7. Installing three light switches in the
basement and swapping all the lamps in the house took our energy usage from 50kw per day to
34kw per day. Because we found this out after installation, we overbought solar.
Smart money buys a kWh meter for your house that shows usage minute by minute. There are
many to be had on amazon in the $150-$200 range. Spending a little on a power meter now
can save you from spending more on solar than you need. A personal favorite is a power meter
called egauge (www.egauge.com). This unit has 16 inputs for current measurement and can
keep one minute recordings of power usage for 30 years. It can also interface with many solar
inverters going forward. For example, it is networked to my inverters and can display battery
voltage and state of charge.
Here is a sample display from the egauge system. It shows solar production in green and
usage in red. We have programmed the system to be able to track production from two different
solar inverters and also to show us heat pump usage and car charging usage. Below we see a
cloudless morning becoming cloudy in the afternoon. The short red spikes are the HVAC
system cycling on and off. The larger red places are two electric cars charging. If you look
closely at the green portion, you’ll see two dashed lines. The higher one is the SolarEdge 12kw
inverter that is on the SW panels and the smaller production is from the SMA Sunny Boy that is
on the SE panels.
We used the egauge to identify a base load in the house. In our case it was three lights in the
basement that were on all the time. We’ve also used to egauge to realize that the horse trough
was left running or the toilet flapper was stuck causing the well pump to rapid cycle. We have
connected current measurement transformers to the feeds for our electric cars and our heat
pump so we know in an instant how much we spent on car charging last year. Having an
internet connected power meter continues to save us money.
Further, once a solar system is installed you can use a power meter such as egauge to verify it
is working. I know of many cases where a family waits for their first electric bill to see how much
power they made only to find out that the installer didn’t turn on all the inverters or there is a
So now that you’ve optimized your energy usage, let’s get back to determining how large of a
solar array you need.
The next question to answer is how many hours of sunlight does your area get each day on
average? You can check an insolation chart or use this one
https://www.altestore.com/howto/solar-insolation-data-usa-cities-a35/ . In our example system, I
find that the nearest city to me is Charleston SC and the average full sun hours are 5.06.
So let’s take the 50kw per day and divide by 5.06 hours of sunlight…. The answer is
approximately 10kw. Congratulations, you now know you need a 10kw solar system.
A typical turn key installed grid tie system cost $3/watt. Recently Tesla has offered turnkey
systems for $1.50/watt. So our example system will cost between $15,000 and $30,000.
As we begin our journey, we start with the panels themselves. Panels are typically 39” wide and
65” long or even longer. An individual panel typically produces 250 – 400watts. If you need a
10kw (10,000watt) system, you’ll need between 25 and 40 panels. Each panel has an
aluminum frame and a clear lexan covering. The panels should survive hailstones and snow
loads for 20 years. They can be mounted in portrait (vertical) or landscape (horizontal) format.
The panels will have a pair of 3 ft wires attached to the back. The idea is that the wires from
each panel can reach its neighbor’s wires. The connector used is MC4 and is completely
waterproof. These connectors should never be plugged or unplugged while the system is
making power. They will arc and the arcing will degrade the connectors. Break the connection
at the solar inverter first, then plug and unplug the MC4 connectors.
Please note that most panels lose ~0.6 to 1% of capacity per year of use. For ease of
conversation we’ll call it 1% a year and accept that at the end of 20 years the system won’t be
broken, it will just be producing 80% of its original power. The smart person includes this in their
Panels can be mounted either on your roof or on a ground mount. With a roof mount you must
use the pitch of your existing roof even if it is not optimal. For example at the latitude of
Columbia South Carolina, the optimal roof pitch is around 8 pitch (8” rise every 12” run). In
upstate New York the optimal angle may be 12 pitch (12” rise every 12” run). In most cases
you’ll probably find that your roof pitch is too shallow. You can make up the lowered efficiency
by simply adding a few more panels.
Here is an example of one type of roof racking system. Each mounting pedestal is screwed to
the roof with a lag screw. A stamped piece of flashing is used to divert water around the
penetration and the hole is typically filled with caulk before the screw is installed. The panels
are then clamped on their edges to the pedestals and to each other.
Building codes will require panels on the roof of a habitable structure to have a 3ft easement
around the sides and top. This is so the fire department can get on the roof safely should there
be a fire and the roof needs ventilation. This 3ft easement also applies to roof valleys but in this
case it is 18” on either side for a total of 3’.
Very often installers must skip a space due to a plumbing vent. Be sure to check with your local
plumber to see if you can replace the vent pipe with an air admittance valve. These are typically
$20 at Lowes or Home Depot and allow proper plumbing venting from inside your attic.
You can also ground mount solar panels. The advantages are that the panels run a little cooler
than on a roof and you can optimize the tilt for your area. The disadvantage is they take up
space in your yard, if you have livestock, cows might push them over when rubbing and
something from your lawn mower might hit them.
Wiring the Panels
The solar panels are daisy chained together using waterproof connectors that are attached to
the back of the panels. Just as a bunch of D cell batteries are in series in an old school boom
box, so do solar panels connect in series. Each panel can be thought of as a battery that
makes 40v. By comparison your car battery is 12v and a D cell is 1.5v. Typically 12-14 panels
are connected in series to make ~560v DC. One series of these panels is called a string. Each
panel makes about 7-9amps. By comparison a coffee maker uses 10-12amps. Because the
current (amps) is so low, the wires from the roof to your inverters are standard 14AWG
household wires. In our 10kw example system we determine that we need 3 sets of 14 panels.
So we will run 3 pairs of 14AWG wire from the roof to the inverter on the side of your house. In
this example, we have 3 strings of fourteen 250w panels for a total of 42 250w panels. The
maximum power this system can produce is 10,500watts.
The astute reader will realize that having 3 pairs of wires with nearly 600v on them could be
dangerous to handle and they would be correct. The national electric code recognized this
problem as well and requ
ires all panels to be disconnected and the system voltage to shut off within 30 seconds of an
emergency. Prior to 2019, the code only required the wires coming from the roof to be
disconnected, but NEC 2017 Rapid Shutdown requires module level disconnection. This is
accomplished by attaching switch modules on the back of each panel or sometimes one per pair
of panels. The inverter system will communicate with these modules either by radio or by
communication signals overlaid on the power wires. Depending on which inverter brand you
use these modules may be called optimizers or they may be referred to as a Tiga system.
The next topic for discussion is that of shade. Back to our old school boom box with 8 D cell
batteries. If one battery goes dead, the boom box won’t work. With solar panels, shade cast on
one panel in a string will kill the entire string in the same way. Solar inverters such as Morning
Star, SMA, Schneider electric have no solution for this problem. The only solution is to try to
wire the panels at risk for shade into their own string. SolarEdge uses optimizers to intelligently
figure out which panel(s) is/are shaded and electrically skip around them. These optimizers
mount on the back of the panels and communicate with the inverter about once a minute. If a
panel is found to be holding back the rest, it is bypassed.
When considering a site for solar you should look for trees, chimneys and TV antennas.
Anything that casts a shadow wider than about 4” could hinder a significant amount of power
production. There is an app for iOS called sun seeker that overlays the path of the sun in the
various seasons of the year onto the camera view. Use this app to locate trees that may cause
As we all know, the sun is higher overhead in the summer and closer to the horizon in winter.
The optimal angle for a solar array facing directly south is that of your latitude. For example,
Columbia South Carolina is at 34 degrees north, so our panels need to be 34 degrees off
horizontal. Locate your latitude on the chart below to see if your roof is at the optimal angle. In
our example, 34 degrees is an 8 pitch roof. Since you cannot control the pitch of your roof, the
only solution is to add more panels to compensate for the reduced production. In our case we
have a 16.5kw system that makes a maximum of 14kw due to our roof pitch being too shallow.
Azimuth is the angle of your solar array as compared to south. If your array faces due south, it
has an azimuth of 0 degrees. If your array faces south west it has an azimuth of 45 degrees.
Naturally there is a sweet spot for the tilt of an array if it isn’t facing due south. A competent
solar installation company can determine this angle for you or you can use an online calculator
to find the correct angle for a given location and azimuth.
For most applications you are confined to the existing azimuth of your roof and the existing
pitch. For ground mount applications, you can control the azimuth and tilt of the panels.
Using the calculator available at https://www.sunnydesignweb.com/sdweb/#/Home , we can
determine that our example 10kw system with 42 panels can make us 15,300 kwh per year. At
12c/kwh, that is $1836 per year. If the combination of roof pitch and house direction isn’t
optimal, it could cost us $150 in electric production per year, or put another way, we would have
to add three additional panels to the roof to make up the difference. In my example, I took the
worst case by comparing a panel mounted facing south at the optimal angle to a panel mounted
on a 12 pitch roof facing south west.
Why wouldn’t you want an array to face directly south if it was possible? In off grid applications
you want to make more power in the morning to recover your batteries as quickly as possible in
the morning. Experience has shown that the South East USA tends to get rain in the
afternoons, so it is advantageous to bias a solar array to make more power in the morning. You
can also split an array into two south east and south west facing arrays. A ratio of 2⁄3 of your
panels south east and 1/3 of your panels south west could spread production out more evenly
through the day with a bias for recharging batteries in the morning.
Grid tie systems should simply be optimized for facing south since their power will be sent into
the grid without the need to recharge batteries or concern for time of production. One may wish
to put more panels to the south east to optimize production before afternoon showers roll in.
Given the price of solar panels, it has become economically infeasible to install trackers. The
cost of maintenance and the outright cost of a tracking device can be more effectively used on
more panels. If more power is needed over a longer portion of the day, split the panels into two
In climates where it rains every few days, there is no need to clean your panels. In dry and
dusty climates you may need to spray them off periodically. Pollen season absolutely affects
performance and we’ve seen peak power drop by 10% due to pollen and rise again the day
after a rain. Things to keep in mind:1)these panels have to last you 20 years so do not scrub
them or even squeegee them. Think about how many tiny scratches you can accumulate in 20
years. 2)Rain and dew are the best cleaners because they are pure water with no minerals.
Tap water sprayed onto hot panels will flash off and leave a film of mineral deposits behind
reducing production. It is probably best to spray the panels with a garden hose just after dark
when they’ve cooled and then allow the dew overnight to rinse off the tap water.
A Photovoltaic (PV) inverter takes 150-600v DC and makes AC power that is synchronized to
the grid. PV inverters come in three varieties: String, Optimizer and microinverter. We’ll cover
each in detail.
Each input on a PV inverter must be between 120vdc and 600vdc. Please note that when
panels are cold they produce a higher voltage and on very cold mornings this higher voltage
could be enough to damage your inverter. For example, your panel’s Voc (voltage open circuit)
is rated at 40v and your PV inverter can accept 600v. 600/40=15 panels seems to be a
workable solution. However on a cold morning that 40v may become 42v and that string is now
producing 630v. The solution is to either calculate the highest Voc voltage for your climate
using information from the cell manufacturer or simply calculate the number of panels in a string
so that the maximum system voltage is 525v. This allows a 75v buffer.
Most solar panels produce less than 10 amps, so inexpensive 14AWG wire is a good choice.
Longer wire runs may dictate a larger gauge wire.
Some lower voltage (150vdc) inverters may allow only 3 or 4 panels in series and accept up to 2
strings in parallel (total of 6 or 8 panels). Since the current from each string is likely to be 7 or 8
amps, two strings will provide up to 16 amps and this will necessitate 12AWG or larger wire.
When designing a 150vdc system pay particularly close attention to VOC at the coldest
temperature ever recorded for your part of the country. Midnite solar charge controllers have a
safety which disconnects the panels from their equipment is the VOC is too high. It monitors the
voltage and when it is back in range will reconnect the panels. The problem occurs on very cold
mornings at exactly sunrise.
In addition to limits on voltage on the inverter, you must also be concerned with maximum power
of the array as a whole. It is customary to exceed the wattage of the inverter by 25%. Keep in
mind the 1% per year degradation of the panels. After 20 years your panels will be producing
20% less power. By exceeding the inverter’s power by 25%, you leave a 5% margin for the
future. For example if you have an inverter rated at 3.8kw, it is perfectly acceptable to feed it
with 4.75kw of panels. The peak production may max out and flat line but there is no harm.
The inverter won’t be harmed, it simply takes what it can from the panels. Clipping as it is called
allows you to produce maximum power earlier in the day and for a longer duration. In the
illustration below, you can see that despite the clipping significantly more power is produced as
compared to an array that matches the maximum inverter power.
MPPT stands for Maximum Point Power Transfer. To understand what MPPT is requires a
basic understanding of electricity. Power = Volts x Amps. For example 32 v x 7amp = 224
watts. But if the voltage goes to 0 and current is 10amps, power is 0. Likewise if voltage goes
to 40 and current is 0, the power is also 0. There is a sweet spot somewhere in the middle.
MPPT is a method for finding that spot.
You can demonstrate this technique with a garden hose. Pressure is equivalent to voltage and
flow in gallons per minute is the equivalent of current. Placing a 1⁄4 turn valve in the end of a
garden hose allows you to vary from no flow to full flow. If there is no flow, water pressure
(voltage) is maximum. If the valve is opened fully, there is full flow and no pressure. By varying
the valve (or your thumb) on the end of the hose) you can quickly find the spot that sprays the
most water the furthest. This is what MPPT does.
Depending on the sun’s intensity and panel temperature that “sweet spot” may move around
from moment to moment. Some inverters move their target panel voltage around and see if the
current goes up or down. This method is called perturb and disturb. Some inverters cease
production for a second and sweep all possible voltage / current combinations every 5 minutes.
If you see a charge controller or PV inverter with a far too low price, it will probably be PWM
instead of MPPT. PWM simply pulses the power from the panels to your battery and does not
optimize for peak power production. They are very inefficient and are to be avoided on all but
the smallest systems.
A string inverter simply accepts a “string” of solar panels and typically has 3 inputs. You can
connect three strings of panels to one inverter (West, South, East for example).
Inverters in this category are SMA Sunny Boy, Schneider, SolArk. As noted earlier, these
strings are particularly at risk for panel shading. One cell of one panel shaded can stop
production of the entire string.
SolarEdge makes inverters which use optimizer modules on the back of each panel. These 4”
square devices connect to the + and – leads of the panels and then using a second pair of wires
form a string of optimizers. The advantage here is that in case of shading any underperforming
panel can be switched out of the string automatically. Each optimizer can report each panel’s
production so if you have a bad panel the SolarEdge app can show you which one it is and
where it is located on your roof.
A microinverter takes the 40v from a single panel and makes 240v AC directly on your roof. It
mounts behind each panel and connects to a 240v cordset run across the roof. The advantage
of this system is that 100% of the electronics are on the roof and do not have to be mounted on
the walls of the structure and the system is not affected significantly by shade. Enphase and
Magna Power make these types of systems. They are generally not acceptable for off-grid use
because 1)enphase does not use a standard method for output limiting, 2)magna energy uses
are proprietary method of output limiting. This will be further explained in the off-grid chapter.
Grid-Tie connection to grid.
So far, we’ve analyzed household electrical usage, speced a system, placed panels on the roof
and selected a PV inverter. All that is left is to actually connect it to the grid.
Despite all inverters adhering to UL 1741 and not making power when the grid is down, all
electric utilities require a lockable disconnect. This is so a lineman can ensure that your system
will not back feed the grid if line maintenance is being done.
You will also need a new electric meter. Modern “smart” digital kWh meters are programmed to
count upwards no matter what. This means that if you simply install a solar system on your
existing meter, your production will be counted as usage despite the meter running backwards.
If you are to backfeed the grid, you need to coordinate and get approval from your local power
company and a “net meter” installed.
A typical install involves turning on the system for about 5-10 minutes to make sure it is working
and then shutting it off until the power company can come install a “net meter”. It typically takes
1 to 3 weeks for the power company to send an employee out to swap your meter.
Residential service is generally limited to 20kw back feed. Some electric companies will cease
approving new solar system installs once they reach 2% of system capacity. The reason for
limiting solar installs is 1) they are in the business of selling power and 2) at mid day the grid
could potentially be flush with power causing them to not need to run a generation station.
Since these stations take 10’s of minutes to spool up, they limit solar contributions to the grid to
a number they can compensate for if the solar were to suddenly drop. Hawaii has exactly this
problem and their island grid is so unstable that the inverter manufacturers use an entirely
different set of specs for that grid. The problem is called the duck curve problem. As solar
power provides significant portions of electrical demand, the power company shuts down
generators and can’t restart them quickly enough as the sun sets and people begin cooking
This is why megawatt scale grid battery backups are so important in recent years. The point of
the battery is not to run the grid but to smooth out supply and demand – to buy them time to
spool up a generating station. Power companies buy and sell power from neighboring grid
operators and the ability to fill in demand with a battery while a generator is spooling up instead
of buying power at an expensive demand price may represent a significant cost savings.
Some power companies exercise a “peak demand” fee on residential customers for both
consumption and production. For example, if your peak usage goes over 10kw for one second,
you will be charged a demand fee for that month. In the case of Duke Progress Energy, the
basic meter fee is $9.83/month plus usage. But if you exceed the demand threshold they will
bill you an extra $8 for the month. This demand fee / penalty also applies to production. If your
production or usage goes over 10kw you’ll also get dinged for the demand fee. If your
production exceeds your usage and your bill would have been $0, you will still receive a bill
each month for $10-$18.
That’s about it for grid-tie systems. In a nutshell your goal is to run the meter backward enough
to offset your electric bill.
When most people think of an off grid system, they picture a cabin in the woods with no
electrical service for miles. While this is possible, the reality is that the grid reaches nearly
every residential address. Being off grid is not a matter of availability, it is a choice.
In an off-grid system your goal is to run your house when and if the grid isn’t present. For some
it may be during a storm when the power is out for a few hours, or it may be to run your house
for a week in the aftermath of a hurricane.
Off-grid systems don’t have to be completely off grid all the time. If the grid is present, they can
sell power just like a grid tie system.
Some may even measure power usage inside the house and never export power but
seamlessly import power if needed. These are called net-zero systems.
Very often an off grid system is not sized to run the entire house. It is common to create a
critical loads subpanel and relocate certain circuits in the house to that panel. Well pumps,
ceiling fans and lights come to mind.
A Word about Power usage.
If the grid tie system all you needed to be concerned about was the amount of power produced
in a year matching your annual usage. In an offgrid system you must concern yourself with
making sure your inverter can supply your peak demand and how many days of batteries you
The calculations for off grid include peak usage (ie how many appliances on at the same time)
and hours or days of reserve power. Peak usage wasn’t of any practical consideration in a grid
tied system because most homes are served by 50kW (ie 200 amp service). Peak power will be
measured in kW. For example running an 1.8kW hair dryer and a 1.2kW microwave oven at the
same time results in a peak draw of 3kW. In this case you must buy an inverter that can supply
more than 3kW or choose to not run both of those loads at the same time.
To illustrate the difference between kW peak and kWh, consider the following: You run the
1.2kW microwave oven 3 times a day for 5 minutes and that is 1.2kW x 1⁄4 hour (15 minutes
usage) = 0.3kWh per day. On the other hand, if you use a 25w reading lamp for 12hours, that
is also 0.3kWh per day.
You will need an inverter that can supply peak power and a battery that will power you for your
anticipated number or days or hours.
A fair analogy is that of water supply. You need to size pipes to allow the peak water flow and
you need to use a water tank that holds enough water for a day’s usage.
Another consideration is the idle power usage on your inverter. Larger inverters at idle will
internally consume more power than smaller inverters at idle. In the case of a 4kw inverter, the
idle power is about 52w. This may not sound like much but over a 24 hour period that is
1.25kWh. If, for example your battery consists of 8 golf cart batteries with 5kWh of energy,
you’ve just used 25% of your battery to keep your inverter humming all day. Consider this as a
water tank with a slow leak.
The solution for high idle current is to simply switch the inverter off when you aren’t using it or
use a large and a small inverter to run different loads in the house. For example, you may run
the microwave and hair drier on a 3kW inverter that you shut off when not in use and use a
300W inverter to run your PC and a reading lamp which stays on all day.
This may be fine for a small cabin, but in a normal home, you would simply size the inverter to
run your peak loads and specify a battery and solar array large enough to cover the parasitic
Another solution is to interlock high demand loads so that two cannot run at the same time. For
example you can interlock your microwave oven with your well pump. A Dwyer current sensing
switch MCS-111050 can be used to send when an appliance is running and pull in a relay that
prevents another device from running.
Yet another solution for a mixed grid system is to move high current devices to a load panel that
is supplied by grid only. For example, it is possible to run a heat pump off grid but running the
heat strips is not a good idea. You can easily separate the heat strips from the electronics and
blower within your heat pump’s inside unit and feed them from grid only. In the author’s case,
the heat strips are tied to a grid only panel. Additionally, the contactor that switches them on
can simultaneously activate gas logs. The idea here is that if the grid is down and the heating
system needs heat strips it can fire up the gas logs.
The author of this article is presently writing an application in Node Red running on a Raspberry
PI to do load management in the house. For example, if the electric cars are charging and the
thermostat is calling for heat, the cars will be commanded to slow or stop charging before the
HVAC system is activated. If other loads in the house are too high, the HVAC system may wait
until the household usage drops below 3kw.
Another way to deal with high capacity requirements while keeping idle power draws low is to
“stack” inverters. Many brands of inverters can be linked so they stay in phase (in sync) with
each other and only switch on as many inverters as power is needed.
Magna Sine inverters are linkable in such a way that one 4kw inverter stays on as the master
with up to 3 additional inverters in hot standby mode. As loads increase in the house, the other
inverters are automatically brought online.
Most other inverter brands simply stack the inverters in parallel for more power and all are on
idling 100% of the time.
Be aware that 240v inverters not meant for the US market will NOT be split phase. They will
produce 240v instead of the 120/240v used in US households.
Food for thought: It is entirely possible to use a 240v inverter to run 120v appliances by using a
step down transformer. This has the advantage of not having to worry about balancing 120v
loads on the two legs of a split phase system. Imagine if the microwave and hair drier are both
powered from the same leg of 120v. It may be possible to overload that one leg while the other
has no load at all. By taking the entire 240v feed through a stepdown transformer to 120v. To
use the step down 120v method, use 240 from the inverter in a conventional panel box to run
stoves, ovens, well pumps and also a step down transformer. The 120v output of the step down
transformer can then be run to a 120v only panel box for all 120v loads in the house.
SMA Sunny Island inverters for the US market make 6kw of 120v and so must be added in pairs
to make 120/240 split phase. To make 120/240 you must use either two or four Sunny Islands
making either 12kw or 24kw.
Another consideration is the system voltage. Very small inverters typically run at 12v. Mid sized
inverters between 1kW and 4kW tend to be 24v. Inverters larger than 4kw are generally 48v. In
practical terms, if you intend to grow your system beyond 4kw, you should strongly consider 48v
inverters even if your system only needs 2kw today. The issue is that current gets so high it
necessitates very large cabling. For example, 4kw at 24v is 166amps and requires AWG 3/0
cable. 4kw at 48v requires AWG 3 cable. More on battery voltage later.
Beware of cheap chinese inverters which claim some high wattage but don’t back it up with
specification. For example, the Sunny Island 6kw inverter says in the specs it can do 7kw for 3
minutes, 8.4kw for 1 minute and 11kw for 3 seconds. A 4kw Aims inverter struggled to provide
DC Coupled Systems
Off-grid systems can be AC or DC coupled. A DC coupled system uses a solar powered charge
controller to charge a battery and the inverter then draws power from the battery to power the
loads in your house. This is what most people first imagine how off grid solar works. The
battery inverter is the primary power source for all loads in a DC coupled system. DC coupled
systems can sell power, but 100% of the power sold must first go through the batteries and
there may be a 15% loss in efficiency in/out of battery. DC charge controllers tend to cost more
than their PV inverters.
In the picture below, the solar panels feed a 48v charge controller (small yellow box) that
charges the batteries. The Sunny Island inverter (larger yellow box) can draw off the batteries
and also recharge the batteries when the grid or generator is present.
DC charge controllers fall into two categories: expensive 150v or 200v units and really
expensive 600v units. Midnite Solar and Morning star make very well respected DC charge
AC Coupled Systems
An AC coupled system works in conjunction with PV inverters and simulates the grid to keep
those PV inverters running the loads in your house. In the day time the primary source of power
for your appliances in the solar PV inverters and the battery inverter is basically there for backup
support if power needs exceed solar production at that moment. Another advantage of an AC
coupled system is that of peak power. If your battery inverter is capable of producing 12kw and
you have 12kw of solar PV inverters, then if the sun is shining, you actually have 24kw of power
available to the house.
The central point of the AC coupled system below is the Sunny Island inverter. The battery
inverter becomes the gatekeeper for the grid interface, a battery charge controller and also
controls the PV inverters.
In the picture below the red Sunny Boy inverter (right) takes DC from the solar panels and
makes AC that can run appliances directly and simultaneously the Sunny Island (left) can use
that power to charge the batteries.
First and foremost, the inverter provides a 240v “lifeline” signal that is very stable to fool the PV
inverters into continuing to run. You may recall from the previous chapter in this article that UL
1741 requires inverters to shut down for 5 minutes if the grid drops. The battery inverter
simulates the grid well enough to keep the PV inverters operational. Should the sun go behind
a cloud or the sun set, the 240v lifeline signal will begin to produce power to run your house.
The battery inverter watches the quality of the connected grid and will sever the link to the grid if
it is out of spec. Just as with the UL 1741 rules, the battery inverter will wait for 5 minutes of
stable grid power before reconnecting. It’s job is to separate from the grid and keep your PV
Another function of the battery inverter is to recharge the batteries. Typically the maximum
charge current to a battery can be in excess of 120amps DC. These inverters have many
settings for bulk, absorption and float charge voltages. In many cases these voltage settings
can be adjusted to be suitable for 24 or 48v lithium batteries.
There is a special case in an off grid system where the grid is disconnected and the batteries
are full and demand is low. In this case the PV inverters must be throttled back. There are two
ways to do this: 1)proprietary communications cable, 2)Frequency Shift Power Control.
In the case of frequency shift power control, consider first that the normal line frequency for
North America is 60hz +/- 0.5hz. If a battery inverter sees the frequency higher than 60.5hz or
lower than 59.5hz, it assumes the grid is unstable and will disconnect. Once disconnected from
the grid, the inverter will then operate the solar PV inverters between 55.5hz and 62hz. The
magic happens between 61 and 62hz. PV inverters complying with this method will throttle
back to making no power at 62hz but will remain in “hot standby”. If the frequency drops any
lower than 62hz, they will immediately make up to 100% of the power they can.
Why 62hz? It goes back to gas powered generators. When you put a heavy load on a
generator, it will slow down below 60hz. When there is no load on a generator it will speed up to
it’s preset idle of 62hz. So a generator making 62hz doesn’t need any additional power and a
generator making just below 60hz needs all the additional power it can get.
SMA Sunny Island inverters maintain a count of the amount of time the frequency was shifted
above 60hz in a 24 hour period and will run the system at a frequency lower than 60hz for an
equivalent amount of time. The purpose of this is to ensure that clocks on your microwave and
stove don’t run fast. Every 24 hours they will be run fast or slow as necessary to reset to the
correct time based on 60hz AC power.
Another method for using excess solar power when demand is low is to heat hot water. Even if
you use an instant hot water heater you can benefit from solar hot water. Simply install a 40
gallon hot water heater inline ahead of your instant water heater. The warmed water will offset
the energy needed to heat your water to the desired temperature. Three 255w solar panels or 6
255w solar panels in series can directly drive a 120v or 240v water heater element. You need
only to swap the thermostat with a thermostat suitable for switching DC such as one sold by
Missouri Solar https://mwands.com/water-heater-thermostat . DO NOT use the standard
thermostat included with the water heater. And always ensure that the emergency popoff
thermostat remains wired into the circuit.
Many battery inverters have programmable relay outputs that can indicate battery State of
Charge and these can be used to divert solar panels to other loads.
Most people will begin a system with 6v golf cart batteries. If you treat them right and never
discharge below 50% they will last about 5 years. You’ll have to add distilled water periodically.
A 6v battery consists of three 2v cells and over time each 2v cell may become imbalanced. The
method used to “balance” lead acid batteries is crude… you overcharge them until the weakest
cell is at the same voltage as all the others. Typically, each cell is charged to 2.6v (63v on a 48v
battery). This causes a great deal of heat and the water boils out of the electrolyte producing
hydrogen gas. This gas must be vented and water level should be checked in each cell.
The plates of golf cart batteries aren’t particularly thick and each equalization cycle erodes them
a little. Eventually all that lead precipitates to the bottom of the battery and shorts out the
plates. L16 batteries have much thicker plates than golf cart batteries and can take more
abuse. The most robust lead acid battery of all is a forklift battery. In forklift service they take
300amp loads continuously and last 5 years. In residential service, they rarely see high loads
and can last 15 years. A 48v forklift battery can have a capacity of 45kwh and weigh 2600 lbs.
They do require distilled water fill touchups once a month and they do require proper ventilation.
The picture below shows a SMA 3.8kw Sunny Boy, a 11.4kw Solar Edge, two SMA Sunny
Islands and a 48v 45kWh forklift battery. The battery was later covered with a ventilated
AGM batteries contain the boiled off water and recycle it back into the cell. They require no
maintenance after the equalization (balancing ) cycle. They also only last about 3 years.
There is nothing magical about lithium battery charging. Unlike a lead acid battery there is no
need for an absorption or bulk charging cycle. You simply push electrons into them and stop
charging when full. The problem comes when you approach 4.2v per cell. Please recall from
above when we discussed balancing lead acid cells by over charging them and boiling off water.
Lithium batteries cannot boil off water so they can only make heat… and catch fire.
A battery is comprised of cells in series. Lithium batteries are routinely described as 6s or 14s.
This means 6 or 14 cells in series. Since each cell is 4.2v that nomenclature tells us it is a
25.2v or 58.8v battery. In the lead acid example above we discussed one cell being weak. The
same problem can exist for lithium. Imagine a string of 6 cells where one is weak. 5 cells are
charged to 4.2v and one cell is at 3.8v. If our charger continues to charge these cells to 25.2v
we will have 5 cells with 4.28v and one with 3.8v. Chances are good that one of those 5 will
catch fire. The solution is a battery management system.
A battery management system will measure the voltage on each cell and stop the charging
process when any cell reaches 4.2v. Some battery management systems will use resistors to
drain the cells with higher charge until their voltage matches the weakest cell at which time
charging can resume. Other more sophisticated systems like Batruim can “shuffle” excess
electrons between cells rather than simply bleeding off thru a resistor.
On the other end of the lithium scale we have to worry about over discharging the cells. Just as
with charging, the battery management system also watches for any cell to get below 3.2v and
stops all drain.
If you ever run a tool battery down too low, it can often be “jump started” by connecting it to a
similar battery to allow it to charge to a level that the charger can recognize as valid and being
The simplest lithium charging system consists of several 40v solar panels in parallel connected
to the battery through a large relay. The panels put out 40v with no load but when connected to
batteries will load down to the battery voltage. In one such system, the designer has 16 panels
in parallel producing 112amps. There is a voltage sensing relay on the battery and when it gets
to 25.2v, it will disconnect the panels from further charging. This system has no BMS and
serves only as a proof of concept. A California man claims to have run his house off this system
with no BMS for nearly a year and only sees 9mv (0.009v) difference between any two cells. I
cannot enphasize enough that there must be some method of detecting when any cell is over
4.2v and stopping charging immediately.
Many people will attempt to use Tesla or GM Volt lithium batteries. The problem with these
batteries is that they are 6s or 12s. An adjustment needs to be made to the inverter/charger to
make sure these cells never charge higher than 25.2v or 50.4v. Additionally most inverters will
shut off for low battery at 21v or 42v. Since 6s or 12s are good down to 18v or 36v, a lot of
capacity goes untouched. The solution is to create at 7s or 14s battery.
For a 24v system the only practical way to get a 7s battery is to make them from individual
18650 cells. There are many groups of facebook which cater to 18650 battery packs such as
For a 48v system you can make the battery from 18650 cells or from 7 Nissan Lead “spam
cans”. Since a Leaf car battery comes with 48 spam cans, buying one additional spam can from
ebay can make a 7s and 7parallel configuration which is very well matched to 48v lead acid
compatible inverters and chargers.
Tesla makes the famous Power Wall. It uses lithium batteries and an inverter in the same box.
While they certainly work and are maintenance free, a handy person can home brew a system
that has 9 times the capacity for the same money.
It can be quite expensive to buy a 200amp transfer switch for your whole house. A more cost
effective way is to install a generator back feed breaker in the top left or right column in your
main panel and interlock that breaker with the 200amp main breaker. You can make a sliding
gate yourself or buy one from https://www.geninterlock.com/ .
Here is an interlock used to select grid or inverter power in the author’s home. This facilitates
bypassing the inverters if they need service.
It is also worth mentioning that many inverters such as SMA Sunny Island have a mode switch
that is usually tied to the transfer switch used to swap in a generator. This mode switch
changes certain parameters in the battery inverter and signals to the inverter that generator start
is available should the batteries get low. The settings in the inverter generally relax the
tolerance for frequency and voltage changes.
Cheap Chinese inverters such as Aims claim to have a generator start function but it does not
work well. The gen start will trigger when the batteries are critically low and your generator will
start. After 30 seconds the inverter will switch the AC input on and begin charging the battery.
Within a few moments the battery voltage will rise above 24 or 48 volts and the inverter will
release the generator start output. The problem with this is that the batteries didn’t really charge
in that short time and now that the generator isn’t running, the batteries are in an even lower
state of charge. This rapid cycling will continue until the batteries are depleted. The solution is
to buy a timer from ebay that once triggered keeps the generator start signal asserted for 1 to 3
Another problem arises when shutting down a generator. As the generator is shutting down and
slowing, it’s on board voltage regulation spike the armature current to keep the output voltage
up. This can burn up the brushes. It is why generator manufacturers advise to disconnect all
loads when shutting them down. Another side effect is that the erratic behavior of the voltage
regulation results in a reactive power alarm on many solar PV inverters. The solution here is to
use a 12v contactor in line with the gen start signal that releases as soon as the generator start
signal is lost. In this way the generator is electrically disconnected at the moment the gen start
signal is lost.
Yet another problem with generators on an off grid system is the interaction with the PV
inverters. Although the frequency shift method is supposed to be compatible with generators,
the PV inverter companies advise against connecting to a generator. The generator
manufacturers also advise against running solar at the same time as the generator. The
solution is to interlock the generator start signal with a relay that disconnects the PV inverter
before the generator can even start. Elkhart / Esco (574) 264-4156 sells a DPDT 50 amp
contactor with an auxiliary switch part number 21082-84. The generator start signal can pull in
the contactor coil and if the PV inverters are connected to 240v mains via the NC (normally
closed) contacts, they will be disconnected. The auxiliary switch can then be used to then
activate the generator start directly. This ensures that the PV is disconnected before the
generator can even start avoiding any warranty voiding back feed power issues.
Financing rip offs and sales pitfalls
Would you buy a car from a buy here pay here car lot? Neither would I. So why would you
entertain a solar company that touts “no money down” or “we’ll pay you to install solar”?
Here’s how the scam works: In South Carolina, you can get a tax credit for 25% state and 26%
federal. A solar company will sell you a system and finance it into two loans. 51% of the price
will be in one loan and 49% in the other. The 51% loan will be a short term with a balloon
payment that must be satisfied when you file your taxes the following year. The problem arises
when, due to your financial circumstances, you aren’t due a large enough refund to cover the
loan payoff. The solar tax credit is a refundable credit meaning that you can only get a refund
on taxes paid this year. If you can’t claim enough tax to offset the payoff on that loan you must
carry over the excess credit until next year. This is the crux of the problem. The loan is due
before you can file your taxes the following year.
The other portion of the loan is structured to be paid in 20 years. That loan will have a lien filed
against your home that must be satisfied before closing if you sell your house. That loan can be
transferred to the new homeowner, but ask yourself… if you were buying a house that came
with solar would you volunteer to assume payments on a loan someone else was responsible
If a system is financed, FHA guidelines do not allow it to be included in the appraised value of
your home. A paid-for system will be included in the appraisal. In short a system with a lien on it
is a liability and a paid off system is an asset.
Paying for solar doesn’t always have to be a bitter pill. Duke/Progress in North Carolina often
offers a cash rebate of $1/PV watt if you use their preferred installers. In the case of a recent
install, the homeowner received a check in the mail for $16,500 about 30 days after their system
was commissioned. Here is an example of an affordable system. The homeowner installed a
16.5kw system for $48000 cash. 30 days after the install Duke Energy sent a rebate check for
$16,500 bringing the system price down to $31,500. At the time of this install the combined SC
and Federal solar tax credit was 55%, so at the next income tax filing the homeowner received
$26,400 in tax refund. This was actually split over two years as a carryover. This meant the
out of pocket cost was $5100. The electric bill had been $1850 per year. The system made
enough power to sell $700 back to the power company the first year, so $2550 was offset from
the power bill. Payback was two years on the nose. Typical payback is 6-8 years.
When talking to solar installers you need to discuss kWh per year of production and dollars per
watt installed. A good starting point until recently was $3/PV watt for a grid tied system. Many
solar installers want to sell systems at $4 or even $5 per watt and offer to replace the light bulbs
in your house with LED or to blow insulation into your attic in addition to the solar install. Why
would you pay extra to have someone do something you could do yourself? Recently Tesla
upset the industry by offering turn key grid-tie systems for $1.50 per watt. Keep in mind that
cash is king… if you need the installer to get financing for you he’s going to charge more per
watt. If paying cash you can bargain with him to stay nearer to that $1.50 or $2 per watt price.
You should expect the salesman to measure your roof area and provide a layout of the panels.
This layout should include 3’ easements around three sides of the panels and 18” on each side
of a valley or dormer. If they want to skip a panel due to a plumbing vent, offer to have a
plumber install an air admittance valve in your attic and remove the vent pipe. The hole can
then be repurposed to feed the wires into your attic.
The salesman should provide you with a model of the estimated production per year that takes
into account your roof pitch and the direction of your roof layout. For example, a 16.5kw system
should produce 30,714kWh per year (16.5 x 5.1hours per day x 365days) but if the roof pitch is
not optimal, it will produce 23,000kWh per year. Make sure you check his model carefully. Do
not let them talk about how many dollars cheaper your power bill will be… keep the
conversation around kWh used and produced.
A reputable solar company will guarantee their estimated production for a number of years after
the install. Fly by night companies will sell you panels they know won’t produce due to shade or
Make sure that the solar company uses their own installers and does not job-out to
subcontractors. There was a case in South Carolina where the salesman bid a job for 25
panels but did not observe the easement requirements. When the contractor arrived on site to
do the install, he realized that they could only fit 21 panels on that side of the roof. His solution
was to convince the homeowner to install the panels on the north side of the roof because it
didn’t have dormers. The system has never made any significant amount of power and the
homeowner is stuck with an electric bill and a loan payment. Later there were leaks and the
sales company directed the customer to contact the subcontractor. The sub wouldn’t talk to
them because they hadn’t paid him directly and the sales company said it fell on the contractor.
Round and round they went. Long story short, use a company that employs its own install crew.
Get your solar installer to give you access to daily production reports from your system. All
solar inverters sold today have the ability to connect to your home wifi lan. You can check
production daily using an ipad. Waiting for the next electric bill can allow a problem to go on far
The talking points in this article were meant to familiarize you with the concepts and terminology
used in residential solar systems. This is by no means detailed enough to design a system but
should be treated as a primer.