If you need just a picture of the whole amp, buy a Kill-A-Watt ... But even with those meters, you can't apply a simple formula like P=E squared over R and get a useful power measurement.

The meter coil has its own tempco of +3930 ppm/°C, which is also simulated here though it makes little practical difference. Just as well: might be tricky to keep it isothermal with the other ...

The folks at [electro-labs] designed this simple USB power meter which does just that ... amplified by the LT6106 a high side current sense amplifier before being fed to the 10 bit analog port ...

This application note shows how to create a simple, low-cost phase meter that provides close to 1% accuracy. To view the application note, click on the Applications such as amplifiers, audio ...