Electric motors are often rated by one of two methods: amperes (amps) or horsepower (HP). Amperes are a measure of the rate of flow of electricity, whereas horsepower is a measurement of work divided by time, so amperes and horsepower cannot be equated or converted to one another (it would be like trying to convert pounds to miles). However, with a little math and one other variable, volts (V), the correlation between amperes and horsepower can be found.
- Calculator (optional)
If the electric motor's placard lists its wattage, you can use that to proceed directly to Step 3 to lessen the workload.
Determine the voltage and ampere ratings for the motor or appliance by looking for a placard on the motor listing these details. Look for numbers with a unit of A or Amp for amperes and V for volts. If no placard can be found, you can guess the voltage by how the motor is powered. If it plugs into the wall of your house, the voltage is 115 V; if it is powered by a car battery, the voltage is 12 V. For example: 5 A blender that plugs into the wall using 115 V.
Multiply amps by volts to obtain the motor's wattage, or watts (A * V = W). Wattage is the same type of unit as horsepower, a measure of power, so it can readily be converted (like gallons to quarts). The wattage for the blender example would be 5 A * 115 V = 575 W.
Divide the wattage by the conversion factor 746 W per HP to convert watts to horsepower. The equation would be as follows: (W) / (746 W per HP) = HP. In our example, (575 W) / (746 W per HP) = 0.75 or 3/4 HP.
Things You'll Need
- If the electric motor's placard lists its wattage, you can use that to proceed directly to Step 3 to lessen the workload.
About the Author
Christopher Perkins is a Gold Seal-certified flight instructor and commercial pilot with a Bachelor of Science degree in aviation. He has held FAA certification since 2005, has been writing since 2007, and has a strong background in physics, mathematics and mechanical systems. Perkins writes for eHow, COD, and Answerbag.