back to getting started with solar

0 quick solar facts:


There's lots of helpful little factoids that you'll eventually run into with solar power. So here's a short
summary of most of the important ones to give you a quick head-start:

For a quick estimate on panels with 4.5" cells, I count the cells and multiply by 2 to get my expected watts.
So when I see a clown on Ali Express describing a 2 x 3 panel (of 4.5" cells) as "100 watts", I know it will
produce closer to 12 watts. (and should be getting listed as a "16 watt" panel, roughly 25% more)

Another way to roughly estimate the rating of a panel (with any size cells) is to measure the entire panel
width and height (in inches) and multiply them to get the square inches, and divide that by 10 to get rated
watts for the panel. (wide panel boarders can throw this off by quite a bit)

Newer cells are "monocrystalline", older cells are "polycrystalline". The older poly panels have a
"kaleidoscope" look to them, the newer mono are one solid color. Poly are less efficient, always get mono.

Quality solar controllers are "MPPT" (maximum power point tracking) and usually include both buck and boost
regulators to trade panel volage and current to the load. The controller chip constantly varies this ratio,
hunting for the ratio that produces the maximum number of watts delivered to the load and battery. MPP is
dependent on both the panel's native maximum power point and the charge state of the battery.

Solar cells do not have a linear voltage-to-current curve, and as load (current) is increased, their voltage
will reach a "knee" on the graph and sharply drop. This means there is a "maximum power point" (MPP) where
an increase in load (current) causes a higher percentage drop in voltage, or a decrease in load produces a
lower percentage increase in voltage - either way leading to a lower power delivery. (in watts) Vmpp doesn't
change with sun exposure, less sun just gives you fewer amps at Vmpp.

Cheap solar controllers are "PWM" (pulse width modulation) and use PWM to force down the panel voltage to
some preset value, usually 13.8 for battery or 5.0 for USB. Solar power produced during the "off" part of
the pulse is thrown away, creating inefficiency when panel voltage exceeds load voltage. Load on the panels
is also not optimal and will produce additional inefficiency, but PWM is cheap and easy to build.

Because most MPPT controllers can boost, they can continue to deliver power even when panel voltage is below
the battery / load voltage. PWM controllers stop harvesting energy when panel volage drops below charge voltage.

When a solar panel refers to its "panel voltage", they are considering each cell to be 0.5 volts. So a panel
with a grid of 4 x 9 cells (36 total) will be referred to as an "18 volt panel".

0.5 volts per cell is a median voltage. Common monocrystalline cells produce maximum power at 0.4 volts, so
a grid of 4 x 9 cells (36 total) will have a "Vmpp" (voltage at maximum power point) of 14.6 volts. With no
load, cell voltage will rise to about 0.625 volts, leading the same 36 cell panel to develop a Voc (voltage at
open circuit) of around 22.5 volts. Your solar controller must be able to tolerate the higher Voc voltage.

Cell (and thus panel) current depends on the size of the cells. The most common cell size is 4.5 x 4.5 inches
which can deliver about 5 amps. Stringing cells in series increases voltage but doesn't increase current.
A typical 36 cell ("18 volt"") panel will produce 14.6 volts at 5.2 amps at its maximum power point in full sun.
(about 76 watts into the solar controler in Iowa) More sun means more current is available at Vmpp.

Solar panels are rated using a fixed amount of light striking them in laboratory conditions. (1,000 watts per
square meter) This is more than hits anywhere on earth, but is a fixed state used to rate and compare solar
panels. Similar to MPG ratings on cars, the user will never be able to attain this value, but it can be used
to fairly compare different panels for relative performance. The above 36 cell panel which produces 76 watts
in Iowa at noon is listed as a "100 watt panel", suggesting 1000 w/m^2 can deliver 6.8 amps per 4.5" cell.

A typical 36 cell panel with 4.5" cells has cells with a surface area of about 20 square inches. 36 of these
would be 720 square inches. There are 1550 square inches in a square meter, so that's 0.46 square meters of
cells. The STC solar test exposes panels to 1,000 watts per square meter of light, meaning the cells were
exposed to 460 watts of light. The STC rated these panels at 100 watts, which is the power they obtained from
the 1,000 w/m^2 exposure. 100 of 460 watts is 21% panel efficiency, which is very close to the claimed 22%,
which is also the commonly quoted current efficiency of "consumer grade" solar cells. So the math checks out.

Commercial house panels often use the same size cells but in a 6 x 10 grid, calling them "30 volt, 150 watt".
A larger common size in use is 8 x 12 (48 volt, 240 watt) panels. Considering the higher DC voltages present,
a home solar installation is often considered a high voltage hazard area. Take steps to keep your solar safe
for visitors on Field Day!

Solar panel efficiency drops as temperature increases. Solar cells tend to be dark and heat up quickly when
exposed to full sun, and must be allowed to cool with airflow to keep operating efficiently. If you protect
your panels by covering them, make sure they have ventilation. (the hot air can be exhausted down into your
basement in the winter, greatly increasing your solar return) As a general rule, every degree over 25c will
reduce solar efficiency by 0.3 to 0.5 percent. It's not a lot, but it starts being a problem if you enclose
your panels without ventilation in the summer.

Solar panel efficiency is also reduced to 75% when angled 25 degrees away from the sun, or 50% at 45 degrees.
However, the headache of dealing with wind on Field Day is usually not worth the hassle of trying to angle.
The additional hardware you need to bring along also greatly reduces portability and increases setup time.
However, panel angling IS helpful in the winter when the sun is low and there's no wind to flip your panels.

Your latitude (distance from the equator) and the season will affect the maximum elevation of the sun in the
sky. This not only affects sun-panel angle, but also less energy is delivered to the panels when the sun is
at a lower point in the sky, due to having to cut through more atmosphere at the steeper angle.

"Solar hours per day" is the average telling you how much solar energy is reaching your spot on earth on the
average throughout the year, where one solar hour is equivalent to high noon sun exposure for one hour. Charts
are available online to look up this value. In central Iowa, we only get an average of 4.5 solar hours per day.
This can be used to calculate how much energy a home solar installation would be expected to produce per year.
Of course this means there will be more solar hours in the summer and less in the winter.

If you have several solar panels, place them in series to increase the voltage without increasing the current
to reduce power loss in your wires running to your panels, but ONLY if your solar controller can tolerate the
higher voltage. Panels can be connected in a combination of series and parallel connections to balance voltage
and current limits. Make sure your panel Voc is safely below your controller's (panel) Vmax.

Oh and those battery banks with the solar panels on them... they're pretty weak. Lets do the math!
A 4.5 x 4.5 cell has 20in2 of area and can produce 2 watts. a 2.5 x 4" cell has 10in2 and so should produce
1 watt. At 4.5 solar hours per IA day, that's about 4.5Wh. An iPhone 13 Pro has a 3095mAh battery, that's
11.5Wh. So that panel will take about 2.5 days (assuming no losses anywhere) to recharge from a single full
phone charge, probably 3 days counting losses.


solar panel testing (link)



last updated 10/11/2023 at 11:18:00