Basic Eqtn - Power(watts) = volts x amps. With power supplies, need to look at the supply to find actual voltage - my Radio Shack is 13.8v and 25amps, so its rated power - 13.8 x 25 = 337.5watts.

My other PS is a ProPeak 20a supply and its also 13.8v but 20a, so its only 276watts. Both of these are switching type supplies versus transformer based PS and its prob prudent not to push these to 100%. Most I've ever done with these is charging 6S a123 pacs - 21.6volts at 10amps => 216watts. Switching supplies have rep for quiting under high load.