Microwave ovens are the most efficient appliance for cooking or reheating small meals.Several studies have shown that if properly used, microwave cooking does not change the nutrient content of foods to a larger extent than conventional heating, and that there is a tendency towards greater retention of many micronutrients with microwaving, due to the shorter cooking time. A microwave oven converts only part of its electrical input into microwave energy.
A typical Smeg microwave oven consumes 1350W of electricity in producing 950W of microwave power, an efficiency of 64%. The other 400W 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 surplus heat, along with heat from the product being microwaved, is exhausted as warm air through cooling vents.