Saving Power

Doing a bit of working out, I've found out that my PC if left on 24 hours uses around 5units (kwh) of power with the monitor and speakers on which is costing around 55p(80c) a day and driving up our electricity bill. I was wondering if you can get low power modes that, if I was to leave it on unattended that would mean I would still be downloading from BT and IRC would stay connected etc?

I've already got the monitor set up for power saving