A heat pump is designed to push heat through a room using forced air. How effective your heat pump is depends on such factors as the size of the room, how much insulation is present, and the number of windows and exterior walls in the area. You can quickly calculate heat pump size by square footage, but it is worth learning how to size the pump accurately to make sure you are getting what you need. There is nothing worse then buying a pump that is too small to do the job, except, perhaps, spending a ton of money on a pump that is far bigger than what you really need.
Measure the length and width of your room. Multiply them to arrive at the square footage. For example, a 10' long by 10' wide room (10 x 10) would have an area of 100 sq ft.
Convert your square footage into the equivalent measurement in tons. Heat pump output is measured in a unit called tons of capacity. 1,000 square feet is equal to 1.5 tons of capacity. A room that is only 100 square feet in size would convert into 0.30 tons. A ton is 907kg, so you would divide 3000 (1.5 tons) by 100 to arrive at 30. Move the decimal point over to represent what percentage 30 is of 1.5 tons (30 x 100 = 3000).
Covert your tons of capacity into BTU (British Thermal Units). Heat pumps are listed by kW size and not tons or BTU, you need to convert to BTU first before converting to kW. Each ton is equal to 12,000 BTU. For a 100 square foot room with a ton size of 0.30 the equivalent BTU is 3,600 (12,000 x 0.30 = 3,600).
Convert your BTU to kW. Each BTU is equal to 0.293 kW. For a 100 square foot room with a BTU capacity of 3,600, the equivalent measurement in kW is 1,054.8. Round that to the nearest whole number, and you would need a heat pump that can produce 1,055 kW per hour to meet your heating needs.
Get specific and get the absolute right size heat pump. Calculate the volume of your room (length x height x width). Identify the "heating factors" involved using the chart listed in the resources. Then, divide the volume of the room by the heating factor identified in the first chart. Finally, divide that by the heating factor identified in the second chart, and you will have the proper kW size of the heat pump needed.
A common error made when figuring out the size of heat pump needed for a multi-room application is that an average is taken of the requirements of all the rooms. Doing this will result in sizing the heat pump too small, and it will not be able to adequately do its job. The additional stress will also cause it to wear out more quickly. Make sure you are accurately accounting for the needs of each room.