The short answer
An 1,800 sq ft house with average insulation needs about a 2.5 tons heat pump in Minneapolis, plus 10 kW of backup heat for nights near -6°F (or 8 kW with a cold-climate model). In Houston the same house needs 3 tons and no backup heat.
The same house in every state
An 1,800 sq ft, one-story house with average insulation and a standard heat pump, at each state's largest city.
| Alabama Huntsville | 3A | 23 | 2.5 tons | 5 kW |
|---|---|---|---|---|
| Alaska Anchorage | 7 | -4 | 2 tons | 10 kW |
| Arizona Phoenix | 2B | 43 | 3 tons | none |
| Arkansas Little Rock | 3A | 25 | 2.5 tons | 5 kW |
| California Los Angeles | 3B | 47 | 2 tons | none |
| Colorado Denver | 5B | 7 | 2.5 tons | 8 kW |
| Connecticut Bridgeport | 5A | 16 | 2.5 tons | 5 kW |
| Delaware Wilmington | 4A | 18 | 2.5 tons | 5 kW |
| District of Columbia Washington | 4A | 23 | 2.5 tons | 5 kW |
| Florida Jacksonville | 2A | 42 | 2.5 tons | none |
| Georgia Atlanta | 3A | 27 | 2.5 tons | 5 kW |
| Hawaii Honolulu | 1A | 66 | 2.5 tons | none |
| Idaho Boise | 5B | 16 | 2.5 tons | 5 kW |
| Illinois Chicago | 5A | 6 | 2.5 tons | 8 kW |
| Indiana Indianapolis | 4A | 9 | 2.5 tons | 8 kW |
| Iowa Des Moines | 5A | 0 | 2.5 tons | 10 kW |
| Kansas Wichita | 4A | 13 | 3 tons | 5 kW |
| Kentucky Louisville | 4A | 16 | 2.5 tons | 5 kW |
| Louisiana New Orleans | 2A | 40 | 2.5 tons | none |
| Maine Portland | 6A | 7 | 2.5 tons | 8 kW |
| Maryland Baltimore | 4A | 19 | 2.5 tons | 5 kW |
| Massachusetts Boston | 5A | 14 | 2.5 tons | 5 kW |
| Michigan Detroit | 5A | 9 | 2.5 tons | 8 kW |
| Minnesota Minneapolis | 6A | -6 | 2.5 tons | 10 kW |
| Mississippi Jackson | 3A | 29 | 2.5 tons | 5 kW |
| Missouri Kansas City | 4A | 9 | 2.5 tons | 8 kW |
| Montana Billings | 6B | -4 | 2.5 tons | 10 kW |
| Nebraska Omaha | 5A | 4 | 2.5 tons | 8 kW |
| Nevada Las Vegas | 3B | 37 | 3 tons | none |
| New Hampshire Manchester | 5A | 9 | 2.5 tons | 8 kW |
| New Jersey Newark | 4A | 17 | 2.5 tons | 5 kW |
| New Mexico Albuquerque | 4B | 23 | 2.5 tons | 5 kW |
| New York New York | 4A | 18 | 2.5 tons | 5 kW |
| North Carolina Charlotte | 3A | 27 | 2.5 tons | 5 kW |
| North Dakota Fargo | 6A | -13 | 2.5 tons | 15 kW |
| Ohio Columbus | 4A | 11 | 2.5 tons | 8 kW |
| Oklahoma Oklahoma City | 3A | 19 | 3 tons | 5 kW |
| Oregon Portland | 4C | 30 | 2.5 tons | none |
| Pennsylvania Philadelphia | 4A | 18 | 2.5 tons | 5 kW |
| Rhode Island Providence | 5A | 11 | 2.5 tons | 8 kW |
| South Carolina Charleston | 3A | 32 | 2.5 tons | none |
| South Dakota Sioux Falls | 6A | -4 | 2.5 tons | 10 kW |
| Tennessee Nashville | 3A | 21 | 2.5 tons | 5 kW |
| Texas Houston | 2A | 37 | 3 tons | none |
| Utah Salt Lake City | 5B | 20 | 2.5 tons | 5 kW |
| Vermont Burlington | 6A | 0 | 2.5 tons | 10 kW |
| Virginia Virginia Beach | 3A | 27 | 2.5 tons | 5 kW |
| Washington Seattle | 4C | 30 | 2 tons | 5 kW |
| West Virginia Charleston | 4A | 16 | 2.5 tons | 5 kW |
| Wisconsin Milwaukee | 5A | 4 | 2.5 tons | 8 kW |
| Wyoming Cheyenne | 5B | 3 | 2.5 tons | 8 kW |
How it works
The formula
We work out the cooling and heating loads the same way as our AC and furnace calculators, size the heat pump on cooling as ACCA Manual S asks, and then see how much of the heating it can carry.
(up to 125% when the heating load is bigger)
output(T) = rated × share at T (60% at 17°F standard, 90% cold-climate)
house loss(T) = heating load × (65 − T) ÷ (65 − design)
balance point: output(T) = loss(T)
backup kW = (heating load − output at design) ÷ 3,412
Assumptions
- Output falls in straight lines from 100% of the rating at 47°F to 60% at 17°F and 45% at 5°F for a standard single- or two-stage unit, and to 90% and 75% for a cold-climate variable-speed unit. Real models vary: check the 5°F and 17°F figures on the spec sheet.
- The house needs no heat above 65°F outside (sun, people and appliances cover it), and its heat loss grows in a straight line to the design load at the winter design temperature.
- Defrost cycles cut output further in damp cold weather; this is not counted.
- Backup strips come in 5, 8, 10, 15, 20, 25 kW; we pick the smallest that covers the shortfall at the design temperature.
Worked example: 1,800 sq ft in Minneapolis
Average insulation, standard heat pump. Summer design 88°F, winter -6°F.
- Cooling load 26,200 BTU/h = 2.18 tons. Heating load 42,500 BTU/h, bigger than cooling, so Manual S allows up to 125%: 2.73 tons, and the largest size that fits is 2.5 tons (30,000 BTU/h).
- At -6°F it delivers about 9,400 BTU/h, so the shortfall is 42,500 − 9,400 = 33,100 BTU/h.
- Backup 33,100 ÷ 3,412 = 9.7 kW → a 10 kW strip heater. Balance point about 28°F.
- A cold-climate model of the same size delivers 18,400 BTU/h at -6°F and needs 8 kW backup.
Too big or too small
- Too big for cooling: clammy summers. Sizing a standard heat pump to the full winter load makes it oversized in summer, so it short-cycles and leaves humidity behind. Variable-speed units handle this much better.
- Too small: big backup bills. Electric strips make heat at 1 unit of heat per unit of electricity, 2 to 3 times the cost of heat pump heat, so a high balance point shows up on winter bills.
- Cold climate? Pick the right type before going up in size: a cold-climate model of the right size usually beats a bigger standard one.
- Mild climate: the heat pump is sized like an AC and usually needs little or no backup.
Questions people ask
What size heat pump do I need for an 1,800 sq ft house?
With average insulation: about 3 tons in Houston, where cooling sets the size and the heat pump covers winter on its own. In Minneapolis it's 2.5 tons, and a standard heat pump needs about 10 kW of electric backup heat on the coldest nights; a cold-climate model needs 8 kW.
Is a heat pump sized for heating or cooling?
For cooling, then checked for heating. ACCA Manual S sizes a heat pump at 95% to 115% of the cooling load so it still dries the air in summer. Where heating needs are bigger, it allows up to 125%, and backup heat or a cold-climate model covers the rest. Sizing a standard heat pump to the full heating load in a cold climate makes it oversized for summer.
What is a heat pump balance point?
The outdoor temperature where the heat pump's output equals the house's heat loss. Above it the heat pump does all the heating; below it, backup heat (electric strips or a furnace) makes up the difference. A lower balance point means less backup heat and lower bills.
Do heat pumps work in cold weather?
Yes, with less output as it gets colder. A typical standard heat pump delivers about 60% of its rated heat at 17°F; cold-climate models deliver about 90% at 17°F and keep heating below 0°F. Check the 5°F capacity on the NEEP cold-climate heat pump list for the model you're quoted.
How much backup heat do I need with a heat pump?
Enough to cover the gap between the house's heat loss and the heat pump's output at your winter design temperature. Electric strips come in about 5, 8, 10, 15, 20 kW. Some codes and utilities also want enough backup to keep the house above freezing if the heat pump fails.
What is a dual-fuel heat pump?
A heat pump paired with a gas furnace instead of electric strips. The heat pump heats down to a switchover temperature (often 25 to 35°F), then the furnace takes over. Use our furnace calculator for the furnace size: it must cover the full heating load on its own.