iMac + uTorrent energy saving
Yes, if the mac sleeps, you can't run any processing. Apple have released as part of Mountain Lion a background system that can run while asleep (PowerNap), but that's not open to other apps, and also needs a laptop with a flash drive. Besides, torrents require a fair amount of processing of the data, lots of network traffic and HD activity.

Your best bet is to use the Energy Saver panel in System Preferences to get the screen and  computer to sleep after short period of time. You can also check the HD option.

Not sure about uTorrent but Transmission lets you prevent computer sleep when there are active transfers, and you can set limits on when to stop the transfer. So, you can have it set to seed a certain amount after it's finished downloading, then remove it automatically, and your computer will then sleep.

As for measuring power, something like this is what you need:

http://www.amazon.com/P3-International-P4400-Electricity-Monitor/dp/B00009MDBU

Plug the iMac into it and it'll tell you the actual power used. There are a bunch of similar devices, but the Kill-A-Watt ones are pretty well known and widely  used.