Around 40 percent of Americans enjoy a cup of hot tea every day, so the question is, is it more energy efficient to heat water in a teakettle or in the microwave? The easy answer is in the microwave. According to Energy Star, an Environmental Protection Agency program, heating a mug of water in the microwave can save up to 80 percent of the energy it takes to do the same task on the stove. Microwave heat waves concentrate solely on what you're warming, and you are heating only what you're drinking instead of a whole kettle of water. The same principle also applies when you're reheating leftover food.