iMac + uTorrent energy saving I want to keep my iMac on tonight and seed/leech torrents. I usually leave it there until, after 5/10 min or so, the iMac turn the monitor off. I can still hear the hd working.

uTorrent has an option which "prevent sleep when transfers are active" so I guess if the iMac goes in sleep mode I can't seed/leech torrents.

Told so, how can I reduce power consumption? And how can I measure the power I'm using now to decide if it is ok or I shouldn't leave it on?

Thanks!
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.