The wattage of a 120-volt motor can be calculated from one of two sources: amps or horsepower. According to "Ohm's Law," watts are a unit of power calculated from the voltage times the resistance, or amps, drawn by the motor. Horsepower was a measurement originated by the Scottish engineer James Watt, who invented the steam engine. In an effort to market his invention's power in familiar terms, Watt devised the horsepower, which uses a straight conversion to watts.
Look for the horsepower or amps of the motor. This information should be readily available on the motor's plaque or the specification section of the owner's manual.
Multiply the horsepower by 746 to calculate watts. As an example, a 0.5-horsepower motor would use 373 watts.
Multiply 120 volts by the number of amps to calculate watts. As an example, a small, 0.2-amp motor would only use 24 watts.
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