Question : Problem: How much does it cost to keep my computer on overnight?

I'm charged 5 cents/kilowatt hour.  

How much electricity does my computer use per hour?

I don't need to know exactly, just roughly.  I think it will be pretty low, like .001 kilowatt/hour, so probably about .001 * $0.05, so about $0.00005/hour * 8 hours, so about $0.0004 to keep computer on overnight.

Answer : Problem: How much does it cost to keep my computer on overnight?

I used a Kill-A-Watt measuring device, and noted that my Athlon64 X2 with 5 hard drives used 120 watts when idle and 200 watts when booting up.  You can use the power calculator to estimate yours: http://www.jscustompcs.com/power_supply/