How To Check the Output Power (wattage) of a Microwave Oven

© 1996-2009 J. Carlton Gallawa . All Rights Reserved Worldwide

Over the years manufacturers have used several different methods to rate the output wattage of microwave ovens. First, there was the traditional method. Then, in 1989-90 came the JIS (Japanese Industrial Standard). Using the JIS method, ovens rated at 700 watts using the traditional method became 750-watt ovens. In 1990-91 the industry changed to the international IEC-705 standard. This pushed the wattage ratings even higher. For example, models rated at 700 traditional watts were instantly turned into 800-watt ovens using the IEC-705 formula.

The following tests do not use the IEC-705 formula. They will provide a suitably accurate measurement of the output power of any microwave oven. Variations or errors in performing this test will produce uncertain results. If the line voltage (from the electrical outlet) is low, the magnetron output will be correspondingly low and vice versa.

Equipment needed:
• Microwave safe container with 1000 mL (1 Liter) gradation.
• Fahrenheit thermometer with a range over 180 degrees F (Amana part # R0157397), or centigrade thermometer (Amana part # M95D5)
• Accurate timer (digital watch, stop watch or watch with second hand) for ovens with a mechanical timer

Procedure Using Fahrenheit Thermometer:

1. Pour exactly 1000 milliliters (mL) of cool tap water into a microwave-safe container. Using the thermometer, stir the water for about 10 seconds, then measure and record the temperature. For accurate results the initial water temperature should be between 60 degrees F (15.6 degrees C) and 80 degrees F (26.7 C).
2. Place the container on the center of the oven cooking shelf. DO NOT leave the thermometer in the container. The cooking shelf (tray) or turntable must be in place and any metal racks must be removed.
3. Heat the water for 33 seconds at full power. Use an accurate digital timer or stop watch if the oven has a mechanical timer.
4. After the heating time is completed, immediately remove the container, stir the water for about 10 seconds, re-measure and record the temperature of the heated water.
5. Subtract the starting water temperature (step 1) from the ending water temperature (step 4) to obtain the temperature rise.
6. To determine the output power in watts, multiply the total temperature rise by a factor of 100.
7. Example:
   1. Starting temperature (T1) = 60 deg F
   2. Ending temperature (T2) = 72 deg F
   3. Subtract T1 from T2: 72 - 60 = 12 (temperature rise)
   4. Multiply temperature rise by 100: 12 x 100 = 1200 watts
8. The test may be repeated to insure accuracy

Procedure Using Centigrade Thermometer:

1. Same as above
2. Same as above
3. Heat the water for exactly 62 seconds at full power. Use an accurate digital timer or stop watch if the oven has a mechanical timer.
4. After the heating time is completed, immediately remove the container, stir the water for about 10 seconds, re-measure and record the temperature of the heated water.
5. Subtract the starting water temperature (step 1) from the ending water temperature (step 4) to obtain the temperature rise.
6. To determine the output power in watts, multiply the total temperature rise by a factor of 70.
7. Example:
   1. Starting temperature (T1) = 16.0 deg C
   2. Ending temperature (T2) = 33.0 deg C
   3. Subtract T1 from T2: 33.0 - 16.0 = 17 (temperature rise)
   4. Multiply temperature rise by 70: 17 x 70 = 1190 watts
8. The test may be repeated to insure accuracy

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In order to evaluate the independent operation of each individual magnetron, the systems must first be isolated. This is accomplished in older models by disabling one side, then performing an output power test on the functioning side (see below); or, in newer models, by putting the oven in the service test mode, then testing each magnetron individually.

Before touching components or wiring:Make sure the oven is UNPLUGGED.
DISCHARGE ALL HIGH VOLTAGE CAPACITORS. Procedure

There is high voltage present, with high-current capabilities, in the circuits of the high voltage section. It is extremely dangerous to work on or near these circuits with the oven energized. DO NOT TOUCH components or wiring while the oven is operating. Use very great caution at all times.

For older models, the procedure is as follows. Observing the above safety precautions, first disable one side by carefully disconnecting one or both of the leads from the primary side of the high voltage transformer. (See the illustration to the right). Set the oven to cook at full power and do an output wattage check as outlined above. Having established the functional status of the one side, unplug the oven, discharge the high-voltage capacitors, re-connect the transformer primary wires, and repeat the procedure for the other side.

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