You can certainly do some math/estimates based on looking at individual component specifications, but I like using a power meter (built in to some PDUs - which is a feature worth having, or you can buy one, they are quite inexpensive).

A system at idle vs full CPU vs full cpu + all disk will produce very different measurements.

Also keep in mind 80% derating - many electrical codes will state that an X amp circuit should only be used at 0.8X on a continuous basis (and the circuit breakers will be sized accordingly).