I run 6 computers for various reasons. As they do "procedures" off and on throughout the day & night, I always left them on and running.
Using the Kill-A-Watt and a low cost software product, I turn each machine on just before their procedure, then off after it runs.
Net result? About $100/month lower electric bill. Now that's proof you can Kill-A-Watt... if you know where to look for them.