Holy cow, I wrote a book!
The powercfg utility
has been around for a while,
but in Windows 7, it gained a little bit more awesome.
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
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.