Does anyone know how to calculate amplifier watts

I have been looking for a simple calculation that would give me the watts output from an amplifier. The only one that I can find that makes any sense to me is “Volts squared, (the output from the amp) divide by the ohms of the speaker = watts” Does anyone know a simpler formula? I know that it is not 100% accurate, but it is close enough for what I need. Or is this incorrect?

Looks like Ohm’s Law to me. How could it be simpler? If you knew the efficiency of the amp, you could measure how much power it consumes and multiply that by the efficiency. That’s probably what I’d do.