How much power does an iMac 27 use in a real world scenario? I decided to find out. I have the iMac 27, with the Intel Core i5 quad core processor, and 4GB RAM. Apple does not offer SSD drives in the iMac, something I really don’t understand.
First thing I did when I got the iMac: take out the Western Digital Harddisk and replace it with a fast and quiet SSD (a Kingston SSDNow V+ 256GB). Result: a completely silent computer.

How much power does this system suck up from the wall outlet? Measurements were taken using the Voltcraft Energy Check 3000, while running OSX (10.6, Snow Leopard) after a cold boot, no applications running.

Display brightness Power usage
100% (15 notches) 141 W
87% (13 notches) 121 W
73% (11 notches) 106 W
60% (9 notches) 93 W
46% (7 notches) 83 W
System sleeps 0 W
System is off 0 W

When the iMac is off, it is truly off (even the USB ports are not powered anymore).
When the iMac is in sleep mode, it also uses 0 Watts (although the USB ports are now still powered). This is awesome!

A non-SSD harddisk consumes about 10 W of power,  you can add that to the numbers above for an accurate estimate of power usage for iMacs-with-a-harddisk.