Why won't my computer go to sleep? Where is the energy going?
powercfg utility has been around for a while, but in Windows 7, it gained a little bit more awesome.
powercfg /energy will analyze your computer’s power consumption and report on things like devices that prevent the computer from sleeping, devices which won’t suspend, and processes which are increasing your battery drain.
Another neat flag is
powercfg /requests which will report on why your computer can’t go to sleep, for example, because it has open files on the network, or because the clown will eat it.