Watts = Volts × Amps
Watts = Volts² ÷ Ohms
Watts = Amps² × Ohms
Use a multimeter to measure volts and amps, then multiply them
Check the device label for voltage and current ratings, then multiply them
For AC power, use a power meter to measure watts directly
For three-phase power, use Watts = Volts × Amps × Power Factor × 1.732
For DC power, use Watts = Volts × Amps