One of the most basic things one needs to know about a panel is how much total power it puts out and at what Voltage and Amps. Even if you don’t have the manufacturer specs, you can still figure this out with a little counting and some multiplication. We use this trick to suss out our competition to see how much they are underpowering their chargers. These are of course estimates and the actual output of your panel will vary based on conditions (sunlight, temperature, angle) and your load.

Determining Voltage:
Each monocristalline cell piece has generates roughly 0.6 Open Circuit Voltage. Assuming the cells are arrayed in series, the total Voltage is simply the number of cells X 0.6. If you count the cells on our 10 Volt panels, you’ll see 21 cells, so the open circuit Voltage is 12.6. Open circuit Voltage is closer to 0.5 Volts per cell, giving us the 10 Volts. Each amorphous cell has an open circuit of 0.8 Volts.

10 Volt 1.3 Watt Panel

10 Volt 1.3 Watt Panel


Estimating Peak Power in Watts:
Measure the total area in square centimeters. Be sure to subtract any area that is not a solar cell. Multiply area by one of the following factors, depending on your material.


Cell Type Efficiency Factor Comments
Monocrystalline ~17% 0.016 These are solid color similar to the ones we have on our panels.
Polycrystalline ~13% 0.012 These are multi-colored, often in shades of blue.
Amorphous ~6.5% 0.006 This is the material of many thin film, flexible panels.

For our 1.3 Watt, 10 Volt Panel, I measured the total cell area as 85.4 square cm. Multiplied by .016, I get 1.36 Watts. Pretty Close!


Estimating Peak Current:
Lastly, you can use P (Watts) = V*I to determine peak current. Divide total Watts by Voltage. In the previous example, 1.36/10 or .136A / 136 mA, again close to our stated specs.

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