How to work out Watts with Solar Panels

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Solar modules only produce their claimed output under 'Standard Operating Conditions'. These have little resemblance to user reality. In a

typical small 12/24-volt system most modules produce about 70% of that apparently claimed. Here's why. Module output is quoted in watts.

One watt is defined as one amp multiplied by one volt, so to produce 80 watts a module operating at 12 volts must, by definition, produce

6.66 amps. In practice it produces about 4.6 amps - and 4.6 amps X 12 volts is only 55.2 watts. The difference is due to a curious rating that

is based on whatever combination of voltage and current gives the highest number - regardless of whether the module can actually be used at

that voltage. Most modules produce up to 21 volts, with maximum current occurring around 17 volts. Multiply 17.1 volts by those 4.6 amps

and there's the 80-watts claimed. But because there are also losses due to heat you'll only get this on top of an equatorial mountain on a cold

day around noon

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