Watts is a unit of energy that measures power conversion. If you can calculate the "watts per square foot," it will permit you to estimate the average electric power usage for things such as lighting, air-conditioning and other electric equipment in a building or a room. It's also a way to compare the energy consumption in different rooms or parts of a building.
Measure the room's width and length. You can also use the floor plans to determine the room's size.
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Convert the room length and width into feet if you're using a floor plan that gives these dimensions in yards or meters. If the length and width are measured in yards, multiply the values by three. If the they are given in meters, multiply the numbers by 3.280839895. For example, a room is 5.6 by 3.2 meters converts to 18.37 (5.6 x 3.280839895) by 10.50 (3.2 x 3.280839895) feet.
Multiply the length of the room by width to calculate the area in square feet. In the previous example, the area of the room was 18.37-by-10.5--or 192.89 square feet.
Obtain energy consumption (in watts) in a room from documentation, specifications or elsewhere. For example, the lighting in the rooms comes from six 75-watt and two 100-watt lamps. That corresponds to 6 x 75 + 2 x 100 = 650 watts.
Divide the wattage consumed in the room by its area in square feet to calculate watts per square foot. In our example, 650 watts divided by 192.89 square feet is 3.37 watts per square feet.