Heat Pump Size Calculator: What Size Heat Pump Do I Need?

A heat pump moves heat instead of making it, so it can heat and cool with the same equipment. Sizing one means balancing two loads. It has to be big enough to cool the house in summer without short-cycling, and it has to heat the house on cold days, when its output falls. This calculator works out both loads for your house and state, picks a size, and shows where the heat pump stops keeping up and how much backup heat fills the gap.

How it works

The cooling and heating loads use the same model as our AC size calculator. Heating adds slab-edge losses and higher winter air leakage, and it takes no credit for sun, people or appliances. The heat pump side works like this:

size          smallest unit covering 95% of the cooling load
            (or of the heating load at the design temperature, if you choose)
output(T)     rated × (1 − slope × (47 °F − T))
            standard:      100% at 47 °F, 60% at 17 °F
            cold-climate:  100% at 47 °F, 75% at 5 °F
heat loss(T)  UA × (70 °F − T)
balance point the T where output(T) = heat loss(T)
backup (kW)   (heat loss − output) at the design temperature ÷ 3,412

Capacity in the cold. Heat pump output is rated at 47 °F and drops as it gets colder outside. We use straight-line curves: a standard heat pump keeps 60% of its rated output at 17 °F, and a cold-climate model keeps 75% at 5 °F, above NEEP’s 70% minimum. Real units vary, and the manufacturer’s extended performance data is the place to check a specific model.

Heating design temperature. It’s the lowest hourly 10th-percentile temperature from December through February in NOAA’s 1991–2020 normals, at the station nearest a major city in your state.

Sizing for heating picks the smallest size whose output at the design temperature covers the heat loss. In cold places that can mean a much bigger unit than the cooling load needs. That works with variable-speed equipment, but a single-speed unit that large will short-cycle in summer.

Worked example

A two-story, 1,800 sq ft house in Minneapolis has good insulation, low-E windows and ducts in the attic. NOAA normals at MSP give a heating design temperature of −9 °F.

• Cooling load: about 18,200 BTU/h, so a 1.5-ton heat pump covers it
• Heat loss: about 35,300 BTU/h at −9 °F (446 BTU/h per °F of indoor–outdoor difference)
• Standard heat pump: at −9 °F it puts out about 4,600 BTU/h. The balance point is about 36 °F, and you’d need about 9 kW of backup heat
• Cold-climate heat pump, same size: 12,000 BTU/h at −9 °F, a balance point near 33 °F and about 7 kW of backup
• Sized for heating, a cold-climate unit has to be 5 tons to cover −9 °F on its own. Most owners in this climate choose a 2 to 3-ton cold-climate model and keep 5 to 7 kW of backup

Heat pump balance point by state: reference table

The same 2,000 sq ft home in every state, with a standard heat pump sized for cooling and the backup heat each type needs at the heating design temperature.

Tips and common mistakes

Choose a cold-climate model where winters are cold. In zones 5 to 7, the difference between a standard and a cold-climate heat pump is often several kW of strip heat on the coldest nights, and strip heat costs about three times as much to run.

Look at the 5 °F and 17 °F capacities. Rated tons only tell you about mild weather. Ask for the model’s output at your design temperature.

Size the backup heat and the electrical service together. A 10 kW strip heater draws about 42 amps at 240 volts. Make sure your panel can carry it.

Keep a dual-fuel option in mind. Pairing a heat pump with a gas furnace lets the furnace take over below the balance point, which can make sense where gas is cheap.

Fix the envelope first. Every BTU of heat loss you remove lowers the balance point. In cold climates, air sealing and insulation often matter more than equipment.