Computers are notorious energy vampires when powered up and sitting idle. If you use your computer for four hours and leave it on the rest of the time unused, it will draw $167.55 worth of energy per year during its idle time. Turn yours off when not in use, especially overnight… even better, plug it into a power strip.
During summer months, computers that are left on also contribute to the load on your home’s air conditioner because of the excess heat they give off. So be sure to be extra conscientious about shutting it down during the cooling season.