Calculate the output power, or wattage, of a microwave oven using a transparent, microwave-safe container filled with water, which can be tested with a thermometer. This procedure makes it possible to measure the wattage on microwave ovens that do not come with built-in output power tests.
First, pour exactly 1 liter of water into the transparent, microwave-safe container. If the container does not have gradation, accurately measure the water in a graduated cylinder before pouring it into the container. Measure the water's temperature in degrees Celsius with the thermometer. Write down this temperature for later reference. If the microwave oven has a global output power limit, set it to 100 percent, making note to return it to its original setting after the output power test is complete.
Put the container holding the water in the center of the microwave plate. Double check that the thermometer is not inside the microwave before proceeding, as microwaving a thermometer is dangerous. Turn on the microwave for two minutes at full power, then quickly remove the container using non-slip thermal mitts.
To calculate the output power, promptly measure the new water temperature. Calculate the difference between the initial and final temperatures, then multiply this amount by 35 to determine the wattage output.