The power of a microwave oven is given in watts. The magnetron, the part of the oven that makes the microwaves, is unlike a gas flame in that it is either on or off; there are no in-between stages. An oven's intermediate settings are achieved by cycling the magnetron on and off. For example, at 50% power the magnetron is on half the time and off half the time. The International Microwave Power Institute has standardized the meanings of the words used for the settings:
Most cookbooks assume an oven with a power of about 650 to 700 watts. (Power and physical size differ; 700-watt ovens are made in a range of cubic inch capacities.) Such an oven boils a cup of water in 2½ to 3 minutes. Unfortunately, there is no trustworthy way of converting cooking times for a 700-watt oven to cooking times for ovens with less power. A 350-watt oven will not take twice as long. For one thing, much of the cooking is done by heat from the already heated parts of the food. The longer the cooking time, the more time this heat has to do its work, even without any further input from the magnetron. So it is best to guess on the low side (you can always zap the food for a few more seconds, but you can't uncook it) and keep good records.
There is a fairly simple way to compare two microwave ovens. Put a liter of room temperature water in a shallow microwavable container. Take the temperature of the water in degrees Celsius. Microwave for five minutes. Stir the water and take its temperature again. Subtract the first temperature from the second. The result is the number of kilocalories of energy the oven transferred to the water. Repeating the test with the same container in a different oven will provide an accurate comparison of their powers.
In some parts of the country the voltage drops in the evening, when people begin using lights and cooking dinner. The lower the voltage, the lower a microwave oven's power. If your oven seems to take longer to cook at meal time than at other times of day, this could be the reason.
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Last revised: 20 December 2002.