Here's another reason to bring your workout outdoors instead of the gym: The treadmill is an energy hog. An average treadmill uses about 600 to 700 watts of energy per hour; that's equal to leaving a few large flat-screen TVs on at the same time. If you spend two or more hours a week walking on a treadmill, the carbon emissions would equal about 1 percent of a car's output. So if it's a nice day, why not go for a walk instead? And whenever possible, unplug the treadmill when not in use, since even in sleep mode, it still uses a small amount of energy.