Heat Pump Capacity Calculator

Why Heat Pump Capacity Drops in Cold Weather

A heat pump is just a reversible air conditioner — it moves heat from outside to inside in winter. The colder the outdoor air, the less heat is available to move, and the harder the compressor has to work per BTU delivered. The AHRI rating is measured at 47 °F outdoor / 70 °F indoor. As outdoor temperature drops, output decreases along a roughly linear curve that depends on the compressor technology.

Standard vs Cold-Climate Heat Pumps

A standard single-stage or 2-stage heat pump retains about 70% of its rated capacity at 17 °F, 55% at 5 °F, and 40% at -5 °F. These numbers are for typical residential splits; individual models vary. A cold-climate variable-speed inverter heat pump (NEEP-qualified, ccASHP) retains 90% at 17 °F, 80% at 5 °F, and 70% at -5 °F. The extra cost of a cold-climate unit pays back quickly in climates where winter design temps are below 15 °F because you don't need as much expensive resistance backup heat.

Balance Point

The balance point is the outdoor temperature at which the heat pump's output exactly matches the house's heat loss. Above the balance point, the heat pump handles 100% of heating. Below, supplemental heat (electric resistance strips, gas furnace, wood, etc.) makes up the difference. For a 36,000 BTU/hr heat pump in a home with 30,000 BTU/hr heat loss at 0 °F, the standard curve puts the balance point around 20 °F — meaning 80% of the heating season the HP handles everything. A cold-climate version pushes the balance point to about 0 °F, eliminating most backup heat use.

Sizing Rules of Thumb

Size the heat pump to the cooling load first, then check that it can handle the heating load at the balance point. In cold climates where heating load dominates, some installers intentionally oversize for cooling up to 125% to extend the balance point lower. Cold-climate inverter units modulate down to 25% capacity, so oversizing them doesn't cause the short-cycle problem that plagues single-stage oversized systems.

Typical Capacity Derating at Common Outdoor Temperatures

Fraction of AHRI-rated 47 °F capacity retained at each outdoor temperature. Interpolated from the capacity curves used by this calculator. These are representative curves — individual models vary by ±5-10%.

Example: a 3-ton (36,000 BTU/hr) standard HP at 17 °F delivers about 25,200 BTU/hr (70% of rated). Same 3-ton rating on a cold-climate inverter HP at 17 °F delivers about 32,400 BTU/hr (90%). At −5 °F the cold-climate unit still makes 25,200 BTU/hr while the standard unit drops to 14,400 BTU/hr and would need backup heat strips.

Frequently Asked Questions

What does 100% of rated capacity mean?

The heating capacity measured at AHRI standard conditions: 47 °F outdoor / 70 °F indoor / 50% indoor RH.

Can I get real numbers for my specific model?

Yes — ask the manufacturer for the capacity tables or check the NEEP ccASHP product list for qualifying inverter models.

What about defrost cycles?

At temperatures between 25 and 45 °F with high humidity, the outdoor coil frosts over and the HP periodically reverses to melt it. During defrost, no heating is delivered (or electric backup runs). Average capacity is ~5% lower than these curves show in frosting conditions.

Do mini-splits follow the same curves?

Ductless mini-splits are typically variable-speed inverter units; most track closer to the cold-climate curve than the standard curve.

How does humidity affect capacity?

Higher outdoor humidity at cold temperatures increases frost formation and defrost frequency, reducing effective output. The manufacturer derating curves already account for standard humidity conditions.