Depends on the printer, obviously; the temperature of the hotend, heating bed, wether there's other bells and whistles like fans, lights, etc...
You can check the specs of the power source, the maximum power rating will give you the absolute upper limit of your power consumption.
For example my printer has a 30A 12V power source, that's 360 Watts maximum; of course it doesn't run at that limit, probably half as much or even less. I know for a fact that the hotend uses no more than 40W, and it's the biggest load in the system.

For a more accurate figure you'll have to measure the Amps drawn from the source by the printer under different circumstances.