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How to Calculate Efficiency Using Amps and Watts

The efficiency of a motor or other appliance is a vital factor in determining whether you should use it or how often you should use it. According to the first two laws of thermodynamics, you can never get more energy out of a system than you put into it, and there will always be some energy loss due to friction and heat. Electrical efficiency is the amount of input energy that is converted into useful work, and you can use a simple calculation to approximate the electrical efficiency of appliances. For example, you can calculate the efficiency of a motor.

Instructions

- 1
  Look on the motor's faceplate for power information. You should see the output horsepower, the current draw and the voltage rating. For example, let's assume this motor has an output of 1.5 horsepower, an input current of 20 amperes and a 60-volt power supply requirement.
- 2
  Multiply 1.25 by 746 to convert the power output. This equation yields 932.5 watts of output power.
- 3
  Multiply the voltage rating by the current to determine the input power. For example, 60 times 20 is 1,200.
- 4
  Divide the output power measurement by the input power measurement to get a ratio and multiply the result by 100 to convert to the efficiency percentage. In this case, the motor converts 77.7 percent of the input power to electrical work. The rest is lost to friction, sound and heat.

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