How to Calculate Thermal Efficiency


Typically expressed as a percentage, thermal efficiency is a rating most manufacturers of thermal energy devices give their products. Refrigerators, air conditioners, heat pumps, boilers and furnaces are some of the common appliances with thermal efficiency ratings. The input to the device is heat and/or fuel, and the output is heat and/or mechanical work. For example, a furnace uses air (heat) and fuel as the input, and heat as the output. You calculate thermal efficiency by dividing the output of a device by the input of said device.

Things You'll Need

  • Calculator
  • Information sheet of thermal energy device
  • Refer to the information sheet for the specific device, then add the amount of heat energy moving from inside to the amount of power coming in. This sum is the total heat energy that moves to the outside.

  • Divide the total heat energy by the power input. This will typically show an infinite number, beginning with a decimal.

  • Move the decimal point two digits to the left. This is the thermal efficiency percentage. Don't be alarmed if the figure is over 100 percent; it most likely will be. This is because these devices move heat rather than creating it, so the amount of heat moved is a greater number than the work put in.

Tips & Warnings

  • For furnaces and boilers, you can skip Step 1 and proceed with the rest of the formula.

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  • Photo Credit Yamini Chao/Digital Vision/Getty Images Ryan McVay/Photodisc/Getty Images
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