Things You'll Need
Area climate data
The BTU (British Thermal Unit) is a standard measure used for rating the energy output of appliances (such as heating and cooling systems) and for measuring energy consumption. One BTU is equal to the energy needed to increase the temperature of one pound of water by one degree Fahrenheit. Calculating the BTUs required to change temperature by a given amount is fairly easy. However, energy consumption depends on several variables, so making estimates of system requirements is more complicated.
Measure the length, width and height of each room in the building. Multiply these together to find the total cubic feet in each room, and then add the room totals together.
Determine the temperature range you need. For example, if the lowest temperatures are normally around 10 degrees Fahrenheit and you want your home to remain at 70 degrees Fahrenheit, the temperature range for heating is 70 degrees Fahrenheit minus 10 degrees Fahrenheit. You can find climate data for the region where you live on the National Climate Data Center website.
Multiply the cubic feet in the building times the number of degrees of temperature change required. For example, a home with 1,500 square feet with standard 8-foot ceilings is 12,000 cubic feet. If you need a 60-degree temperature change, multiply 12,000 times 60 (720,000). One BTU will raise temperature 1 degree per 55 cubic feet, so divide by 55 (720,000/55). In this example it requires 13,091 BTU to change the building's air temperature by 60 degrees.
Estimate the actual system heating or cooling capacity needed. Heat is lost (or gained in hot weather) through walls, roof and windows as well as the movement of air into or out of a building. For example, a 1,500-2,000 square foot home (with standard ceilings) may require a cooling system of 30,000 BTU capacities. However, unusual exposure to sun can add 10 percent (3,000 BTU). You should add another 600 BTU for each occupant. With 4 people, that's another 2,400 BTU, for a total of 35,400 BTU.
A house that is well insulated and that does not leak air through cracks, windows or doors will require much less heating or cooling capacity. Energy efficient appliances also require less energy, further reducing the BTU requirements.
Determining the rate of energy loss from ventilation or leakage through walls and windows may require a professional evaluation. However, the cost of this service is likely to more than pay for itself. You’ll be able to better select the system you need and the inspection will identify problems (like leaky doors) that you can have remedied, reducing energy usage and costs.