What Wattage Means
Inside a microwave oven, a device called a magnetron generates radio waves with short wavelengths that travel through the oven's cooking area. Food in the oven absorbs these radio waves, and the waves' energy heats the food. A microwave's output wattage rating refers to the relative power of the waves that the oven's magnetron is capable of generating; in general, the higher an oven's output wattage, the faster it will be able to cook food. Once you know your microwave's output wattage, you'll be able to determine appropriate cooking times for your particular model.
The output wattage of microwave ovens varies considerably from model to model, but larger ovens are typically more powerful than smaller ones. Compact microwaves often output between 600 and 800 watts, and mid-sized and larger models commonly output between 850 and 1,650 watts. Manufacturers offer models with a range of output ratings; two well-known appliance manufacturers make microwaves that range from 700 to 1,200 watts.
Where to Look
Most microwave oven owner's manuals include a list of specifications in which the oven's output wattage is specified, and some ovens display the wattage on a label inside the oven.
The wattage will also be included on a label on the back of the oven. This label typically contains details about the oven's manufacturer, its model and serial numbers, its Underwriters' Laboratories registration and any required warnings. Do not confuse the output wattage with the oven's input wattage, which will also likely be included on the label and is a specification of the electricity required to run the oven, not its cooking power.
If your microwave's manual and labels are missing or damaged and you're not able to determine your oven's wattage from them, you may be able to estimate the unit's power output by conducting a simple test. Fill a microwave-safe measuring cup with exactly 8 ounces of cold water and put the cup in the oven. Set the oven to cook for 4 minutes at high power and watch through the window so that you can see when the water begins to boil.
The faster the water boils, the higher your oven's wattage. A 1,200-watt microwave will boil the water in less than 2 minutes, an 800-watt microwave in about 2 1/2 minutes, and a 700-watt microwave in about 3 minutes.
After completing the boil-water test, leave the cup of water in the microwave to cool down, as the water can be scalding hot. If spilled, it can severely burn you.