This site was at the top of the Google results for "How efficient is a microwave oven" I can't restart the old discussion, so I'll start a new thread. Previous posters came up with numbers from 99% to 46%. The Wikipedia page offers only this: "A microwave oven converts only part of its electrical input into microwave energy. A typical consumer microwave oven consumes 1100 W of electricity in producing 700 W of microwave power, an efficiency of 64%. The other 400 W are dissipated as heat, mostly in the magnetron tube. Additional power is used to operate the lamps, AC power transformer, magnetron cooling fan, food turntable motor and the control circuits. Such wasted heat, along with heat from the product being microwaved, is exhausted as warm air through cooling vents." Also of course, some energy exits into the room through the door's grid glass. It's not a perfect barrier, but a convenient compromise. Commercial food service units have no glass. A member "cdotter" offered the observation "Wouldn't the efficiency depend on the type of food? A more polar molecule will rotate better than a less polar molecule." I agree that efficiency also depends on what's in the oven. Presumably if running it empty the oven would still consume house current and yet nothing's on the turntable to get hot. I'm wondering if there is any data about this. 99% 64% 46% is such a wide range.