Heat Pump Size Calculator
This heat pump size calculator sizes a whole-house system in tons, then checks heating capacity at 47 F and 17 F, the balance point and the backup heat needed.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Heat pump size
- 3.5 tons
- 42,000 BTU per hour nominal, 95% of the 44,000 BTU per hour cooling load for 2,000 sq ft
- Heating capacity at 47 F and 17 F
- 42,000 and 27,300 BTU per hour
- Rated capacity is measured at 47 F. This standard unit holds about 65% of it at 17 F, and 51% at 5 F
- Balance point
- 29 F
- Below 29 F the house loses heat faster than the unit makes it, so the backup starts running. A right-sized system usually balances between 25 F and 35 F
- Heat needed at the design temperature
- 52,000 BTU per hour
- 800 BTU per hour per degree F across a 65 degree difference. The heat pump delivers 21,420 BTU per hour at 5 F
- Backup heat
- 10 kW
- 30,580 BTU per hour short at 5 F, which is 8.96 kW. A 10 kW kit draws 41.7 A at 240 V, and NEC 424.3(B) adds 25% for continuous load, so it lands on a 60 A circuit
- Before you buy
- Manual J, then the capacity table
- Planning estimate only. IRC M1401.3 wants a Manual J and a Manual S selection, and the winter answer comes from the manufacturer's extended capacity table or the NEEP list, never the nameplate
Heat Pump Size formula
Cooling load = Area (sq ft) x zone factor (20 to 30) x envelope factor; Tons = load / 12,000
Capacity at temperature T = Rated 47 F capacity x (1 - (47 - T) x (1 - f17) / 30)
House heat loss at T = Area x UA per sq ft (0.28 to 0.55) x (70 - T)
Backup kW = (Heat loss at design - Capacity at design) / 3,412Worked example
A 1,600 sq ft house in IECC zone 6 with a tight shell, a -5 F design temperature and no backup heat, so a cold climate model. The cooling load is 1,600 x 21 x 0.85 = 28,560 BTU per hour, which is 2.38 tons and rounds to 2.5 tons, or 30,000 BTU per hour rated at 47 F. A cold climate unit holding 85 percent at 17 F loses 0.5 percent of rated capacity per degree below 47 F.
Heat loss is 1,600 x 0.28 = 448 BTU per hour per degree F, so at -5 F the house needs 448 x 75 = 33,600 BTU per hour. The heat pump makes 30,000 x (1 - 52 x 0.005) = 22,200 BTU per hour there. The balance point works out to 14 F, and the house is 11,400 BTU per hour short on the design day. That gap is why even a cold climate install usually carries a 5 kW strip kit for the two or three coldest nights.
Quick reference
| Outdoor temperature | Standard unit | Cold climate unit | House heat loss |
|---|---|---|---|
| 47 F | 42,000 BTU | 42,000 BTU | 18,400 BTU |
| 35 F | 36,120 BTU | 39,480 BTU | 28,000 BTU |
| 25 F | 31,220 BTU | 37,380 BTU | 36,000 BTU |
| 17 F | 27,300 BTU | 35,700 BTU | 42,400 BTU |
| 5 F | 21,420 BTU | 33,180 BTU | 52,000 BTU |
Size to cooling, then check the winter
A heat pump has to cover two loads with one compressor, and those loads peak in opposite seasons. Cooling comes first because it has no backup: if the unit is too big for summer, nothing rescues the humidity, and if it is slightly small the house runs a degree warm on the worst afternoon. Manual S allows the cooling capacity to reach 115 percent of the cooling load, and 125 percent where heating governs, so that is the ceiling.
Then check heating. Take the house heat loss per degree, which runs about 0.28 BTU per hour per degree F per square foot for a tight shell and 0.55 for an old one. Multiply it by the difference between 70 degrees and the ASHRAE 99 percent design temperature, then compare the answer to what the unit still makes at that temperature. Whatever gap is left is the backup, and it is normal for the gap to exist. Sizing the heat pump to close it entirely gives you a compressor that is two sizes too big for July.
Capacity falls as it gets colder, and the balance point is where it matters
Rated heating capacity is measured at 47 F outdoors. As the outdoor coil gets colder there is less heat in the air to move, so output drops in a nearly straight line. A standard air source unit is down to roughly 60 to 70 percent of rated capacity at 17 F. A cold climate model with a variable speed compressor holds far more, and the ENERGY STAR cold climate designation requires at least 70 percent of rated capacity all the way down to 5 F. That difference is the whole argument for the more expensive machine in zones 5 through 7.
The balance point is where the falling capacity line crosses the rising heat loss line. Above it the heat pump runs alone. Below it the strips or the furnace fill in. A 28 to 32 degree balance point on a house in Chicago means resistance heat runs for a small share of the season, which is what keeps the electric bill reasonable. Set a dual fuel changeover at the balance point rather than at a round 40 degrees. And get the numbers from the manufacturer's extended capacity table or the NEEP cold climate list, because the nameplate says nothing about 5 F.
What this calculator leaves out
The heat loss here is one UA number for the whole house, so it cannot see which rooms are short, how much glass faces north, or what the ducts lose in an unconditioned attic. It does not model defrost penalty, part-load efficiency, or the way a variable speed unit modulates, and it does not compare running cost against gas at your utility rates. Not included: the duct changes a heat pump usually needs, because it delivers supply air at 95 to 105 degrees instead of 130. Nor the 240 V circuit, the disconnect and strip kit wiring, the condensate drain and pan, the outdoor stand that keeps the coil above the snow line, permits, or any panel upgrade the strips force.
Code limits that apply
- IRC M1401.3checked
Equipment sized by ACCA Manual J and selected by Manual S
If you miss it: An oversized heat pump short cycles in cooling, and the permit stalls where the load sheet has to be filed
- IECC R403.7
Heating and cooling equipment sized to Manual S from Manual J design loads
If you miss it: The energy inspection is held until the contractor produces a load calculation for the house as built
- ACCA Manual S
Heat pump cooling capacity up to 115% of the cooling load, 125% where heating governs
If you miss it: Past 125 percent the summer cycles get too short to dehumidify, and the winter gain does not repay it
- NEC 424.3(B)checked
Fixed electric space heating is continuous: conductors and breaker at 125 percent
If you miss it: A 10 kW kit pulled through a 50 A breaker trips on the coldest night, exactly when the backup is needed
- NEC 424.19
A disconnecting means for the heater, its controller and any overcurrent device
If you miss it: No lockable disconnect at the air handler means the tech has to kill the panel to pull a strip element
- NEC 440.14
Disconnect within sight of and readily accessible from the outdoor unit
If you miss it: The electrical rough fails, and every future service call starts with a trip to the basement panel
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- Sizing to the heating load in a humid climate. The extra tonnage short cycles all summer and the house never dries out.
- Taking the nameplate tonnage as winter output. Pull the extended capacity table or the NEEP listing for 17 F and 5 F.
- A 10 kW strip kit on a 50 A breaker. NEC 424.3(B) makes it a continuous load, so it needs 60 A and the right conductor.
- Dual fuel set to switch at 40 F. The furnace takes over while the heat pump is still the cheaper source at most utility rates.
- No allowance for defrost. Near 35 F the unit reverses for several minutes every hour or two and the strips cover the room.
- Reusing the old air handler. The AHRI match, the blower capacity and the strip kit ampacity all change with the new outdoor unit.
Key facts
- A heat pump's nameplate heating capacity is measured at 47 F outdoors, and a standard air source unit delivers only about 60 to 70 percent of it at 17 F.
- ENERGY STAR's cold climate designation requires an air source heat pump to deliver at least 70 percent of its rated heating capacity at 5 F.
- Electric resistance backup converts at 3,412 BTU per hour per kilowatt, so a 10 kW strip kit adds 34,120 BTU per hour of heat.
- A 10 kW strip heater draws 41.7 A at 240 V, and NEC 424.3(B) treats it as a continuous load at 125 percent, which puts it on a 60 A circuit.
- The balance point is the outdoor temperature where heat pump output equals the building's heat loss, and on a correctly sized system it falls between 25 and 35 F.
Frequently asked questions
What size heat pump do I need for a 2,000 sq ft house?
About 3.5 tons in IECC zone 5 with average insulation, which is 42,000 BTU per hour at 47 F and 27,300 at 17 F. At a 5 F design temperature the house needs 52,000 BTU per hour, so a 10 kW backup kit covers the gap.
How much capacity does a heat pump lose at 17 degrees?
A standard air source unit is down to about 60 to 70 percent of its 47 F rating at 17 F. A cold climate model holds roughly 85 percent at 17 F and at least 70 percent at 5 F, which is what the ENERGY STAR cold climate designation requires.
Do I need backup heat with a heat pump?
In zones 4 and colder, almost always. Even a cold climate unit sized to the cooling load usually falls short on the design day, and every air source heat pump needs heat during defrost cycles. Strip kits come in 5, 8, 10, 15 and 20 kW.
What is a good balance point for a heat pump?
Between 25 and 35 F on a correctly sized system. Much above 35 F means the heat pump is too small or the house is too leaky, and the strips will run through most of the winter. Below 25 F usually means the equipment is oversized for cooling.
Sources and references
- Cold Climate Air Source Heat Pump List, capacity at 47 F, 17 F and 5 F (Northeast Energy Efficiency Partnerships)
- Air Source Heat Pumps Key Product Criteria, including the cold climate designation (ENERGY STAR)
- NFPA 70 National Electrical Code, Article 424 Fixed Electric Space-Heating Equipment (NFPA)
- Chapter 14 Heating and Cooling Equipment and Appliances, Section M1401.3 Sizing, 2021 IRC (ICC)
Next steps for this project
The usual order for a insulation & hvac project. See all insulation & hvac calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.