Originally Posted by
Camilo
amps X volts
seriously, like others said, with a power meter.
Is it possible to calculate it (on a bike) without a meter?
Yes, if you record all the relevant variables (this is essentially what the iBike power meter does, although it's far from perfect).
Edit: the relevant variables being things like: speed, wind speed, rolling resistance, body position, gravity, gradient, etc. etc.