Well, you know the definition of Watts equals Volts times Amps! The only issue is that a 100 Watt panel represents its ideal power output under test conditions. That means that the light intensity and the angle of incidence is ideal. I have attached a spec label of a '100W' Kyocera panel. You will see that the rated wattage is at a light intensity you are unlikely to achieve here on earth due to the atmosphere. They list a more realistic output of 73W at a lower light intensity and a higher cell temperature. That is approximately a 29% reduction and neglects the angle of incidence, clouds, haze, any shading, dust on the panel, actual ambient condition, etc, etc. I have read that the actual output of a panel over several hours is probably closer to 25-30% of the rating.
Also, another point of trivia is that a solar panel is better thought of as a constant current device rather than a constant voltage device. The reason is that the panel will tend to output a similar voltage regardless of the light conditions (but it will vary with temperature). However, if you measured current you would tend to find that the current output is almost directly proportional to the light intensity. The better MPPT Controllers for charging batteries from solar panels try to maximize the Voltage and Amp points to thereby maximize the Power output.
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