If it's an aluminum and glass built iMac (instead of white plastic), Apple has full reports of typical power usage of the 2009-2010 models here:

http://www.apple.com/environment/reports/

With your computer being custom built, it's hard to say. The best way to monitor it would be something attached to the electrical feed to measure it there. Or, depending on the power supply, monitoring software may work to show the realtime usage there. My Mac Pro for example breaks down power usage by component and I can see this via a program (mac only) called iStat.

I am suspicious though of a claim of 1200KHW over 60 days (20 per day average) if your only seeing 5-7 a day by monitoring it recently. Thats more then double, and I'd imagine you didn't just recently decide to turn off a bunch of things.