multiply panel watts × average hours of sunlight ×
75% (as it is unlikely to be at full efficiency):
250 watts × 5 hours × 0.75 efficiency
= 937.5 daily watt-hours.
And then convert it to kWh like our electricity bill:
937.5 / 1000
= ~0.94kWh per solar panel.
Your panel will usually tell you its voltage output,
but that indicates the performance of the panel
in ideal conditions. A great exercise is to take
your panel outside, connect a multimeter to the
panel’s positive and negative leads, and turn the
meter to the voltage reading setting. As you
move the panel, you can clearly see how the
voltage output is being affected (Figure
E
).
You can do the same thing to see the amperage
output of a panel.
Most solar panels have no amperage rating.
Your circuit will draw current measured in amps
(or milliamps, depending on the scale of your
circuit). The consumption of amperage over time
is measured in amp-hours (or milliamp hours).
You’ll typically see this on batteries which have
an Ah or mAh rating, so you may be familiar with
the concept. Why is this important? Because
most solar circuits, as we’ll see below, use a
controller to charge the battery while the circuit
draws power from the battery, instead of the
panel directly powering the circuit. So you’ll want
to calculate the draw of your circuit, and then find
a battery that charges faster than your circuit
discharges it.
Again, it’s important to remember that your
solar panel output is charging your battery, and
it’s your battery output that has to match your
circuit requirements. For example, if my battery
has 500 milliamp hours, and my LED draws 20
milliamps, my battery will last for 25 hours at full
brightness:
Battery capacity (mAh) / Current draw (mA)
500 / 20 = 25 hours
Don’t forget to make sure you’re always working
in the same units, and convert if needed by
moving the decimal point.
You can connect a solar panel directly to the
power and ground wire of any simple electronic
component to power it with the right voltage
(Figure
F
). But be carefuI: If the device has a
processor or controller on it, the fluctuations
in power might cause it to fail or to break.
This is why we use tools to moderate the flow
of electricity and ensure it doesn’t under- or
overpower our electronics as the output of the
panel changes.For example, while my panel
might output 5V, my battery may only output 3.7V
in a circuit with a controller.
SOLAR TOOLBOX
After you’ve decided what exactly you want to
power, you’ll need to find a solar panel. They
come in all sizes, but I’m going to focus on simple
electronics rather than powering your home, RV,
or boat.
Adafruit sells a wide variety of panels, from
a tiny 5V 40mA badge, #700 (Figure
G
) to
larger 6V panels in a variety of wattages and
Lee Wilkins, Adafruit, SparkFun
77
make.co
A 5V solar
panel
connected
directly to a
5V DC motor
using alligator
clips.
F
G
H
M82_074-80_SquishyTech_Lee_F1.indd 77M82_074-80_SquishyTech_Lee_F1.indd 77 7/12/22 10:43 AM7/12/22 10:43 AM
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