Three Ratings, Three Different Things Being Measured
Heat pump spec sheets carry three efficiency numbers that get casually lumped together but actually measure three distinct things: COP measures instantaneous heating efficiency at a specific outdoor temperature, HSPF2 measures seasonal average heating efficiency across an entire heating season, and SEER2 measures seasonal average cooling efficiency across an entire cooling season. Confusing these — or assuming one number tells you what another measures — is one of the most common misreadings of heat pump spec sheets, and it matters because a heat pump's heating-mode and cooling-mode performance are genuinely separate figures, not two views of the same number.
COP: Coefficient of Performance
COP is the simplest of the three conceptually: it's the ratio of heat energy delivered to electrical energy consumed, at a specific, defined outdoor and indoor temperature condition. A COP of 3 means the heat pump delivers 3 units of heat energy for every 1 unit of electrical energy it consumes — a direct measure of the "leverage" the refrigeration cycle is providing, since (as explained in how heat pumps work) the heat pump isn't generating that extra energy, it's relocating heat that already existed in outdoor air. Manufacturer spec sheets typically publish COP at a handful of standard test conditions (commonly 47°F and 17°F outdoor air temperature, per AHRI test standards) because COP is not a single fixed number for a given unit — it's a curve that varies with outdoor temperature, and citing "the COP" without specifying the outdoor temperature it was measured at is technically incomplete.
Why COP Falls as Outdoor Temperature Drops
This is the single most important heat-pump-specific concept to understand about efficiency ratings, and it's the reason COP behaves so differently from a furnace's AFUE. As outdoor temperature drops, two things work against the heat pump simultaneously: there's less accessible heat energy in the colder air for the outdoor evaporator coil to extract, and the temperature difference the compressor has to work against (between the cold outdoor air and the warm indoor target temperature) grows larger, requiring more compressor work per unit of heat delivered. Both effects push COP downward as it gets colder outside. A heat pump might show a COP around 3.5–4 at 47°F but only 2–2.5 at 17°F, and continue declining from there — which is the underlying mechanism behind the concept of a balance point, the outdoor temperature at which the heat pump's declining capacity can no longer keep up with a building's heat loss at all. A furnace's AFUE, by contrast, doesn't follow this pattern — combustion efficiency is roughly independent of outdoor temperature, so a furnace's rated efficiency is a reasonably constant number regardless of how cold it gets outside.
HSPF2 and SEER2: Seasonal Averages, Not Peak Numbers
Because COP varies continuously with outdoor temperature, it's not a convenient single number for comparing equipment models or estimating a season's total energy cost. HSPF2 (Heating Seasonal Performance Factor 2) solves that by averaging heating performance across a full simulated heating season under a standardized regional climate profile, expressed as total heating output (in Btu) divided by total electrical energy input (in watt-hours) over that season. SEER2 (Seasonal Energy Efficiency Ratio 2) does the equivalent for cooling mode. The "2" in both labels reflects the DOE's 2023 revision to the test procedures (M1 test standard, with more realistic external static pressure assumptions than the earlier SEER/HSPF ratings), which is why SEER2/HSPF2 numbers on current equipment aren't directly comparable to older pre-2023 SEER/HSPF ratings without a conversion — a slightly lower SEER2 number can represent essentially the same real-world equipment as a higher pre-2023 SEER number, because the test got more conservative, not because the equipment got less efficient.
Typical modern heat pump equipment lands in roughly 15–16+ SEER2 for cooling and 7.5–8.5+ HSPF2 for heating at baseline efficiency tiers, with high-efficiency variable-speed equipment reaching notably higher — SEER2 in the low-to-mid 20s and HSPF2 above 10 are achievable on premium cold-climate-capable systems. These seasonal figures are useful for equipment shopping and rough operating-cost estimation; COP remains the more useful figure for understanding performance at a specific design condition, such as verifying capacity at a region's winter design temperature.
Converting Between HSPF2 and COP
HSPF2 and COP use different units — HSPF2 is Btu of heat output per watt-hour of electrical input, a mixed-unit ratio, while COP is a dimensionless ratio of energy to energy in the same units. Converting between them requires accounting for the unit conversion factor between Btu and watt-hours (1 watt-hour = 3.412 Btu) and is not a simple one-to-one mapping, since HSPF2 is a seasonal average across many operating conditions while COP is a single-point value. The site's SEER2/HSPF2 to COP converter handles this conversion directly and provides a typical peak/rated COP range with efficiency-tier context, which is the practical way to translate a spec-sheet HSPF2 or SEER2 number into an estimated COP for load calculations or operating-cost comparisons.
Using These Ratings Correctly
For comparing heat pump models against each other for a purchase decision, HSPF2 and SEER2 are the right numbers — they already account for a full season's worth of varying operating conditions in one figure. For engineering verification that a specific unit will deliver adequate heating capacity at a specific site's coldest design temperature, COP at (or interpolated near) that specific outdoor temperature is the more directly relevant figure, since a strong HSPF2 seasonal average can still mask a unit whose capacity and COP fall off steeply at the coldest end of the range — precisely the scenario a proper balance point calculation is designed to catch.