I just got through reading an article regarding buying surplus solar panels and measuring output in watts. The article was completely wrong and made me cringe so I thought in the interest of education I’ll elaborate on why so hopefully people will avoid making this mistake.

People with a little electrical knowledge know that watts = volts × amps. This article suggested calculating the power output of a 12 volt solar panel by measuring the short circuit current and multiplying by 17, because 17 volts is usually approximately the open circuit voltage of a 12 volt panel under full sunlight.

This does NOT yield the power a solar panel can produce, the voltage will decrease as the load increases. Load the panel to the point where the load decreases the voltage to 12 volts, measure the current at that load, and multiple that current by 12 volts. The short circuit current will be higher than the current at normal load, and the unloaded voltage will be higher than the voltage at a normal load, but you get neither of those let alone both at the same time, under load, so those numbers are meaningless. The numbers that have meaning are those under normal load.