V*I, though note that for AC power systems, these are vector quantities. and will require you to determine the "real" portion of this product.

In terms of work, it's Joules per second.





In terms of electromagnetism, it's the product of Volts and Amps.

depending on what you know(have measured)

P = voltage * current = E*I

P = current^2 * resistance = I^2R

P = voltage^2/resistance = E^2/R

I*E