Late to the party, but Maplin were knocking out mains wattmeters for £8, and I'd always wondered what my always-on media server (VIA Epia PD, 600MHz C3, two 3.5in drives, full-size CDROM) consumed. And the answers are:

Plugged in but switched off: 5W
Peak current, switch-on (spinup both drives): 88W
Steady full load (rip+encode+record DVB): 75W
CPU busy, drives spun-up but idle: 62W
Idle, both drives spun-up: 58W
Idle, both drives spun-down: 45W
Idle, both drives spun-down, CPU@400MHz: still 45W (i.e., on-demand CPU governor makes no detectable difference)

Frankly, that's more than I was expecting; perhaps the (200W?) PSU isn't very efficient at that low load. Maybe I should get one of these.

Peter