Depending on your undisclosed specification power supply, it can be between 0.9w to 1.7w An easy test method is plug your PC to a Kill A Watt and monitoring the power usage while the PC is shutdown (WOL enabled) 
How much power does an Inspiron desktop consume in Wake on LAN mode? Simple question I can't find an answer on. In general and on Average (allowing for +/- platform differences) ...With Wake on LAN activated (and it works) and the PC is "Off", what is the quiescent power consumption in watts ?
How much power does an Inspiron desktop consume in Wake on LAN mode?