From MetroAmp Wiki
To get a quick and dirty measurement of amplifier output wattage, without consideration for THD, you can use this basic method.
Thanks to Watt's Law, output in watts can be determined by the formula:
P = V2 / R for peak output in watts.
Multiply P by .707 to calculate output is watts RMS.
To apply this to guitar amp output, you need a known load (8 ohms for example) and a peak to peak voltage measurement at the output.
Ideally, you should send a 1k sine wave into the amp at approx 20-100mV and measure the largest clean P-P sine wave at the output using an oscilloscope. In a pinch, you can measure the AC voltage with a multimeter but this is not as accurate.
Insert your voltage reading and fixed load value into the equation:
P = (30V * 30V) / 8Ω
P = 900 / 8
P = 112.5W Peak
P RMS = 112.5 * .707
P RMS = 80.7 Watts