How to Calculate the Wattage of a 120V Motor

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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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