How to Calculate AC

Calculate the BTU usage of each room, one room at a time. You'll need to know the room measurements, the size and position of the windows and whether or not they use blinds, the number of intended occupants, the equipment in the room and the lighting.

Measure the room's length and width in meters then multiply by 337. Our example room measures 4 by 6 meters, so we'll multiply those figures together (equals 24) and then multiply that by 337 to come out with a final room BTU of 8,088.

Multiply the window measurements (in meters) by 870 if you have a north-facing window, by 165 if the window faces south and 515 if it faces east or west. Multiply that measurement by 1.5 if the window doesn't use blinds. Calculate for each window and add them up. Our example room uses blinds, and has a north and an east window; both measure 1 meter high by 0.75 meter (75 centimeters) wide. So, we'll multiply 0.75 (1 x 0.75 = 0.75) by 870 for the north window (equals 653) and by 515 for the east window (equals 386) and then add them together for a total window BTU requirement of 1,039.

Multiply the number of intended occupants in the room by 400. Our example room is a two-person bedroom that will periodically see a third occupant, so occupant BTU comes out to 1,200.

Add up the wattage ratings of all of the electrical equipment in the room and multiply by 3.5. Our room has a 500-watt plasma-screen TV (500 x 3.5 = 1,750 BTU), a 25-watt DVD player (25 x 3.5 = 87 BTU), a 40-watt portable phone (40 x 3.5 = 140 BTU), a 10-watt night light (10 x 3.5 = 35 BTU) and a 300 watt fan (300 x 3.5 = 1,050 BTU). Added together, this room's equipment thermal load equals 3,062 BTU.

Add up the wattage of the primary lights in the room and multiply by 4.5. This room has two 80-watt light bulbs and a 25-watt reading lamp for a total lighting wattage of 185. Multiply 185 by 4.5 and we have a total lighting thermal load of 832 BTU.

Add all of the single-room BTU figures together, then repeat for every room in the house. For our example room, we'll add 8,088 (room area load), 1,039 (window load), 1,200 (occupant load), 3,062 (equipment load) and 832 (lighting load) for a total BTU figure of 14,221 BTU for this single room. After totaling up all of the other rooms in our example home, we find that another bedroom requires 14,550 BTU, the bathroom requires 6,463 BTU, the kitchen requires 21,250 BTU, the living room 25,925 BTU and each of the home's three closets require 400 BTU (1,200 total). Adding these figures, we find that this home requires an air conditioner with a rating of approximately 69,059 BTU.