HSPF2 & COP explained

By Francesco Zinghinì

Heat pumps do not make heat — they move it, pumping warmth from the outdoor air into your home. That is why their efficiency can exceed 100%: a good unit delivers three or four units of heat energy for every unit of electricity it consumes. Two ratings describe this: COP, an instantaneous measure, and HSPF2, a seasonal one. This guide explains both and how to convert between them.

COP: the instantaneous measure

COP (Coefficient of Performance) is the ratio of heat delivered to energy consumed at a single operating point, in consistent units:

COP = heat output ÷ energy input

A COP of 3 means the unit delivers three units of heat per unit of electricity — equivalent to 300% efficiency compared with a resistance heater’s COP of 1. From a nameplate you can compute it directly: COP = BTU/h ÷ (watts × 3.412). A unit delivering 36,000 BTU/h while drawing 3,517 watts has a COP of 36,000 ÷ (3,517 × 3.412) = 3.0. The COP-at-temperature calculator does this from your unit’s published capacity and power figures.

Why COP falls when it gets cold

The catch is that COP is not a constant — it drops as the outdoor temperature falls. The colder the outside air, the less heat there is to extract and the harder the compressor must work, so a heat pump that hits COP 3.5 at 47 °F might fall to COP 2 near 17 °F, and lower still below that. This is central to sizing and to the balance-point decision, because at some outdoor temperature the heat pump becomes more expensive to run than a gas furnace. Modern cold-climate heat pumps hold their COP far better than older models, but the physics always pushes it down as temperature drops.

HSPF2: the seasonal measure

Because COP varies, you need a seasonal average to estimate a winter of heating — that is HSPF2 (Heating Seasonal Performance Factor, DOE 2023 revision). It is the total heat delivered over a heating season (BTU) divided by the total electricity consumed (watt-hours), so its units are BTU per watt-hour. Like SEER2, the "2" marks the 2023 DOE test procedure with more realistic static pressure, which lowered the numbers versus the old HSPF for the same equipment. Typical HSPF2 ratings run from about 7 for basic units to 10–12 for high-efficiency cold-climate models.

Converting HSPF2 and COP

The two ratings are on different scales but a documented approximation links them, because 1 watt-hour = 3.412 BTU:

HSPF2 ≈ COP (seasonal) × 3.412, so COP ≈ HSPF2 ÷ 3.412

An HSPF2 of 10.24 corresponds to a seasonal COP of about 3.0. This is an estimate, not an exact identity — HSPF2 already bakes in defrost cycles, backup-heat use and temperature variation that a single COP does not — but it is close enough to move between the two for planning. The COP ↔ HSPF2 converter applies it both directions.

Turning the rating into a bill

Seasonal heating electricity is the heat you need divided by the seasonal efficiency: kWh = heating load (BTU) ÷ (HSPF2 × 1,000). A home needing 20 million BTU of heat per season at HSPF2 8 uses 2,500 kWh; upgrading to HSPF2 10 cuts that to 2,000 kWh — 500 kWh saved. Multiply by your own $/kWh rate to value it, and use the HSPF2 savings tool for the full estimate. For running cost at a chosen COP, the heat-pump running-cost calculator uses kW = capacity ÷ (COP × 3,412).

Backup heat and the seasonal average

HSPF2 quietly accounts for something a single COP hides: supplemental heat. On the coldest days, when the heat pump’s capacity and COP both fall, most systems call on electric-resistance backup (or, in a dual-fuel setup, a furnace) to make up the shortfall. That backup runs at COP 1 — expensive — and its use is baked into the seasonal HSPF2 figure, which is one reason HSPF2 lands well below the mild-weather COP. It also means two homes with the same heat pump can see different real-world efficiency: the one in a colder zone leans on backup heat more, dragging its effective seasonal performance down. A cold-climate heat pump earns its premium precisely by holding capacity and COP at low temperatures, so it calls on backup far less often.

This is why sizing and the balance point matter so much for heat pumps: the goal is to cover as many heating hours as possible at the high mild-weather COP and minimize the hours spent on COP-1 backup. The COP-at-temperature tool lets you see how far down the temperature scale your unit stays efficient before backup takes over.

The practical takeaway: use COP when you care about performance at a specific temperature (sizing, balance point, a cold snap), and use HSPF2 when you care about the whole season (annual cost, upgrade payback). Keep the "2" ratings on their own scale — never compare an HSPF2 number against an old HSPF one.