How to Calculate Energy Loss Through an Open Window

Common sense dictates that leaving your windows open on a cold day will result in a loss of heat energy, potentially raising the cost of heating your home. Exactly how much energy is lost may surprise you. Using a simple math equation you can find the hourly loss in British thermal units or BTUs, allowing you to determine whether it would be better to simply pull the window shut.

Instructions

- 1
  Measure the width and height of the open window's aperture. Multiply the two numbers to find the area in square feet.
- 2
  Find the Fahrenheit temperature inside the room with a thermometer. Use an outdoor thermometer to find the current temperature outside the window. Subtract the outside temperature from the inside temperature.
- 3
  Multiply the area of the window by the temperature difference. This is the number of BTUs lost every hour through the open window, with one BTU equal to the heat energy put off by a kitchen match. Divide the result by 3.412 to find the energy loss in watts.

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