Watts = Volts * Amps

You can assume that the voltage is going to be around 115 Volts in North America.

Most measuring devices, like the Kill-A-Watt, will give you the Watt reading directly (because it's also measuring voltage).

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris