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Heat Pump Sizing and Balance Point Calculator

See where a heat pump’s falling capacity meets your home’s rising heat loss, how much backup heat is left at the design temperature, and when a gas furnace becomes the cheaper heat.

Method reviewed October 25, 2026. Version 1.0.0. How it works

Your inputs

Unit system

From a load calculation at your design temperature (for example our Furnace Sizing or HVAC Load estimate, or your contractor’s report).

Your local 99 % heating design temperature.

The building needs no heat above this outdoor temperature because of internal and solar gains. 65 °F is the usual degree-day base.

Maximum capacity from the manufacturer’s extended performance table.

Result

Thermal balance point
25.8°F
Heat pump capacity at the design temperature
16,400BTU/h
Share of design heat loss covered
41%

Balance point 25.8 °F. At 5 °F the heat pump covers about 41 % of the 40,000 BTU/h heat loss, leaving 23,600 BTU/h (6.9 kW) for backup heat.

  • Below 17 °F the capacity is extended from the 47–17 °F line. Real capacity at very low temperatures is often different — use the manufacturer’s 5 °F (or lower) maximum capacity if it is published.
Backup heat needed at design
23,600BTU/h
Backup electric heat at design
6.92kW
Building heat loss per °F (UA)
666.7BTU/h·°F
Show the working
  1. Building heat-loss line: load(T) = UA × (no-load temperature − T)
    UA = 40,000 ÷ (65 − 5) = 666.7 BTU/h per °F
  2. Heat pump capacity line through the manufacturer points
    47 °F → 36,000 BTU/h; 17 °F → 22,000 BTU/h
  3. Balance point: capacity(T) = load(T)
    T = 25.8 °F, where both are 26,118 BTU/h
  4. Capacity at design and backup heat
    16,400 BTU/h vs 40,000 BTU/h → backup 23,600 BTU/h = 6.92 kW

What the balance point tells you

A house loses more heat as the weather gets colder, while an air-source heat pump delivers less. Plotted against outdoor temperature, the two lines cross at the thermal balance point. Above it, the heat pump can carry the whole house; below it, something else — electric strips, a furnace or a second unit — has to make up the difference.

This calculator draws both lines. The building line runs from your design heat loss at the design temperature down to zero at the temperature where no heating is needed (65 °F by default). The heat pump line runs through the capacities you enter from the manufacturer’s data at 47 °F and 17 °F, and at 5 °F if you have it.

Use the right capacity figures

Northeast Energy Efficiency Partnerships (NEEP), whose cold-climate heat pump guide is the reference for this method, recommends using the maximum capacity from the manufacturer’s extended performance data — not the nominal rating on the box. Nominal 47 °F ratings are measured at a speed that is often well below what a variable-speed unit can deliver.

Be careful mixing data sources. ENERGY STAR’s listed 47 °F and 5 °F capacities come from different tests, so pairing them can move the balance point by several degrees. When you have only two points, the calculator extends the 47–17 °F line below 17 °F and tells you so.

Dual-fuel switchover

If the home has a gas furnace as backup, there is also an economic balance point: the outdoor temperature where heat from the heat pump costs the same as heat from gas. Turn on the switchover option and enter the heat pump’s COP at 47 °F and 17 °F, your electricity and gas prices, and the furnace AFUE. Above that temperature the heat pump is the cheaper heat; below it, the furnace is.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Design heat loss
40,000 BTU/h
Outdoor design temperature
5 °F
Outdoor temperature with no heating load
65 °F
Heating capacity at 47 °F
36,000 BTU/h
Heating capacity at 17 °F
22,000 BTU/h
Do you have a 5 °F capacity?
No — extend the 47–17 °F line
Also find the dual-fuel (cost) switchover?
No

Result

Thermal balance point
25.8 °F
Heat pump capacity at the design temperature
16,400 BTU/h
Share of design heat loss covered
41 %
Backup heat needed at design
23,600 BTU/h
Backup electric heat at design
6.92 kW
Building heat loss per °F (UA)
666.7 BTU/h·°F

Balance point 25.8 °F. At 5 °F the heat pump covers about 41 % of the 40,000 BTU/h heat loss, leaving 23,600 BTU/h (6.9 kW) for backup heat.

Assumptions

  • The building heat loss falls in a straight line from the design heat loss at the design temperature to zero at the no-load temperature (65 °F by default, the usual degree-day base).
  • Heat pump capacity changes in a straight line between the manufacturer points you enter, and the end segments are extended beyond them.
  • Defrost losses, cycling and duct losses are not modelled separately. Use capacities that already include them if your data does.

Limitations

  • Use maximum-capacity figures from the manufacturer’s extended performance data, as NEEP’s sizing guide recommends. ENERGY STAR’s listed 47 °F and 5 °F capacities come from different tests, so mixing them can move the balance point.
  • A straight load line is an approximation. Solar gains, occupancy and wind change the real load from hour to hour.

Frequently asked questions

What is a good heat pump balance point?

There is no single target. A lower balance point means less backup heat. In a cold climate, a heat pump chosen to cover the whole design load has a balance point at or below the design temperature. A heat pump sized mainly for cooling may balance at 25–35 °F and rely on backup heat below that.

How much backup heat do I need?

At the design temperature, the backup must cover the heat loss minus what the heat pump still delivers. The calculator shows that shortfall in BTU/h and in kW of electric heat. Electric strips are sold in steps, so choose the step at or above the shortfall.

Should I size a heat pump for heating or cooling?

It depends on the climate and what you want it to do. NEEP’s guide describes sizing to the heating load when the heat pump is meant to replace the existing heating, and checking that it does not oversize the cooling. In hot climates the cooling load usually decides.

Where do I find the capacity at 17 °F or 5 °F?

In the manufacturer’s extended performance tables or submittal sheet, usually listed by outdoor temperature with minimum, rated and maximum capacity. NEEP’s cold-climate heat pump list also publishes 5 °F and 17 °F capacities for listed models.

Sources

  1. Guide to Sizing & Selecting Air-Source Heat Pumps in Cold Climates (rev. 04/08/20) — NEEP (hosted by U.S. DOE Building Science Education Solution Center). Size from the manufacturer’s extended performance (maximum capacity) data; ccASHP maximum capacities at 5 °F can corroborate it; avoid outdoor units whose minimum capacity exceeds the smallest indoor unit’s need.
  2. ENERGY STAR Certified Heat Pumps (dataset 83eb-xbyy) — U.S. EPA (public domain). Certified systems with SEER2, EER2, HSPF2, heating capacity at 47/17/5 °F and COP at 5 °F; 47 °F and 5 °F values come from different tests.
  3. Calculating 5 °F heat pump performance — U.S. DOE. COP = heating output ÷ (3.412 × input watts).
  4. Energy explained — British thermal units (Btu) — U.S. Energy Information Administration. 100,000 Btu per therm; 1,036 Btu/ft³ natural gas; 91,452 Btu/gal propane; 138,500 Btu/gal heating oil; 3,412 Btu/kWh.

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