If you’re boiling, roasting, or baking a dish that will cook for an hour or more, don’t bother preheating your oven. Even for breads and cakes, never preheat for longer than ten minutes. If you reduce the amount of time your oven is on by one hour per year, you’ss save an average of two kilowatt-hours of energy. If 30 percent of U.S. households could each reduce total oven preheating time by just one hour per year, the sixty million kilowatt-hours of energy saved could bake a dozen cookies for every American.