1.03 watts at 110 volts according to this document:
Different models will differ slightly, but by less that a watt (for example, the 2009 iMac uses 1.75 watts).
Hi, I can no longer find the official Apple specs, but I'm reasonably certain that is where captfred got these numbers...
Power Consumption for 27-inch iMac
Mode 100V 115V 230V
Off 0.85W 0.88W 0.90W
Sleep 1.44W 1.42W 1.53W