Find roughly how many BTU per hour an air conditioner or heater needs to keep a room, or a whole house, comfortable.
Describe the room to estimate the cooling or heating capacity it needs.
Heat loss or gain through the walls and ceiling for a given temperature difference between inside and outside.
A BTU, or British thermal unit, is a measure of heat. One BTU is about the heat needed to warm one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTU per hour, which tells you how much heat they can move in an hour. One BTU per hour is about 0.293 watt, and 12,000 BTU per hour is called one ton of cooling.
The calculator starts with a base of 20 BTU per hour for every square foot, plus 2,500 BTU per hour. Ceilings taller than 8 feet add more, and larger rooms add more still. The result is then multiplied for poor or good insulation, for a very sunny or shaded room and for a hot or cold climate. Each person beyond the first two adds 600 BTU per hour, a kitchen adds 4,000, a living room adds 1,000, and a second floor adds 10 percent because heat rises.
This method adds up the surface area of the walls and the ceiling, which is the floor area plus the area of the four walls. It multiplies by the temperature difference you want to hold and by a number for how well the house keeps heat in: 0.24 for good insulation, 0.40 for average and 0.84 for poor. Use the difference between the hottest or coldest day and the temperature you want inside.
A unit that is too small will run all day and never catch up. A unit that is too large will switch on and off too often, which wastes power and does a poor job of removing moisture. Round up to the nearest size sold, and have a professional check the load for a whole house, since windows, doors, and air leaks matter a great deal.
A common rule of thumb is about 20 BTU per hour for each square foot of floor space, and this calculator adds a base amount and adjustments for the ceiling, insulation, sun, climate, people and room type.
One ton of cooling is 12,000 BTU per hour.
Round up to the next common size. Slightly oversizing is fine, but a unit that is much too large will cycle on and off and handle humidity poorly.
Multiply by 0.293. A 10,000 BTU per hour unit is about 2,930 watts of heat moved.
The whole house method works for heating or cooling, since it is based on the temperature difference. The room method is aimed at cooling but is also a reasonable guide for heating.
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