While idling, my main machine draws about 124-125 watts; as soon as I crank up Folding the consumption rises to 150-151. So, running Folding on this machine is about the equivalent of a 25W light bulb. Your mileage will vary.
I keep the machines running 24x7 so consuming a little more power is not a big issue.
Even better - a test with a Kill-A-Watt meter. Has that device actually helped lower consumption?
Funny thing about electricity - I still turn off lights in empty rooms (except for some flourescents) but my computers run 24/7. No wonder my kids are a mess!
As you say, it is about the same as leaving a single bulb on overnight...If cost is the issue, turning your computer on and off is very expensive 'cause it shortens you computer life. The electrical start-up surge to the motherboard as the 'puter is booted-up is VERY destructive. That's why most geeks and many large corporations leave their boxes on 24/7.