A living area's BTU load is the number of BTUs required to heat or cool it. BTU is the usual abbreviation for British thermal unit, a unit of heat energy used to measure the heating and cooling capabilities of air conditioners and furnaces.
An appliance that closely matches your BTU load will provide the best service.
Why Load Calculation is Important
Knowing your home or office's BTU load is a must when purchasing a new heater or air conditioner. An appliance with a rating much below the load level would have a hard time keeping the temperatures at ideal levels.
An overly powerful air conditioner, however, can have equally detrimental effects, short-cycling by cooling the room too rapidly and then shutting off. This doesn't give the AC time to dehumidify the room properly, leaving too much moisture in the air.
A furnace that is too powerful will also burn fuel inefficiently and cause excess pollution.
A living area's BTU load can be roughly calculated using simple mathematical formulae. Multiply the square footage of the room by 3125 to find the area BTU load.
Multiply the square footage of any northward-facing windows by 1524 and those of other windows by 8064, multiplying the product of this equation by 14 if the windows are unshaded to find the total window BTU load. Multiply the usual number of occupants by 600 to find the occupant BTU load.
Finally, multiply the wattage of any office equipment by 34 and that of all light fixtures by 425 to find their BTU loads. Add all five numbers to calculate the total BTU load of the room or building.
Other Means of Calculation
If math isn't your strong suit, you can just as easily find the BTU load by other means. Consumer Reports provides an online tool that takes even more variables into account than the manual calculation, but automates the actual computation, giving you more accurate results for less work.
Other similar websites also exist. For most accurate results, however, hire an air conditioning or heating professional to determine the exact BTU load of your home or office.