SEER2, HSPF2, EER2, and COP Explained: How to Read Heat Pump Efficiency Ratings

SEER2 measures how efficiently a heat pump cools over a season, HSPF2 measures how efficiently it heats over a season, EER2 measures cooling efficiency at a single hot-day condition, and COP is the raw ratio of heat out to electricity in at one moment. All four are worth understanding, but for most U.S. homeowners HSPF2 (and, in cold regions, the COP at 5°F) matters more than the SEER2 number that dominates sales brochures.

The four ratings, one at a time

SEER2: Seasonal Energy Efficiency Ratio 2

SEER2 is the total cooling delivered over a simulated season (in BTU) divided by the total electricity consumed (in watt-hours). A 16 SEER2 unit delivers 16 BTU of cooling for every watt-hour, averaged across a range of mild and hot days. It is the number that determines your summer electric bill, and it is the number most brands advertise. Higher is better.

EER2: Energy Efficiency Ratio 2

EER2 is a snapshot rather than a season: cooling output divided by power input at a fixed 95°F outdoor temperature with the unit running flat out. It tells you how a system performs on the hottest afternoons, which is why the Southwest region attaches an EER2 minimum to its standards. If you live in Phoenix or Las Vegas, EER2 deserves as much attention as SEER2.

HSPF2: Heating Seasonal Performance Factor 2

HSPF2 is the heating equivalent of SEER2: total heat delivered over a simulated heating season divided by electricity used, including the electricity burned by defrost cycles and backup resistance strips when the test conditions call for them. A 9.0 HSPF2 unit delivers 9 BTU of heat per watt-hour on average. Divide HSPF2 by 3.412 and you get a rough seasonal average COP; 9.0 HSPF2 works out to a seasonal COP near 2.6.

COP: Coefficient of Performance

COP is the simplest and most honest number: heat output divided by electricity input at one specific set of conditions, with no units to convert. A COP of 3.0 means three units of heat for one unit of electricity. Manufacturers and the NEEP cold-climate database publish COP at 47°F, 17°F, and 5°F, and the 5°F figure is the one that separates true cold-climate machines from the rest. For the physics behind why COP is above 1, see how heat pumps work.

What changed in 2023, and why the numbers dropped

On January 1, 2023, the U.S. Department of Energy switched residential air conditioners and heat pumps to a new test procedure (known as Appendix M1). The old procedure tested ducted equipment against very little airflow resistance, which flattered real-world performance. The new procedure raises the external static pressure roughly fivefold to mimic actual ductwork, and it adjusts several other assumptions in the heating test.

The result is that identical hardware scores lower under the new rules. Across the industry, SEER2 runs about 4.5% below SEER, EER2 about 4% below EER, and HSPF2 roughly 15% below HSPF. Nothing about the equipment got worse; the yardstick got more honest. This is why a 2021 spec sheet showing 16 SEER describes essentially the same efficiency as a 2024 sheet showing 15.2 SEER2.

Federal minimum standards by region

The DOE divides the country into three regions for central air conditioners: North, Southeast, and Southwest. Air conditioners face different minimums in each. Heat pumps, however, have a single nationwide standard because their heating benefit applies everywhere.

ENERGY STAR sets a higher voluntary bar. For ducted split heat pumps the current specification requires roughly 15.2 SEER2 and 7.8 HSPF2, and the separate cold-climate designation requires an HSPF2 of at least 8.1 (ducted) or 8.5 (ductless), a COP of at least 1.75 at 5°F, and at least 70% of rated capacity at 5°F. The ENERGY STAR Most Efficient tier goes higher still; we track those models in our coverage of ENERGY STAR Most Efficient heat pumps for 2026.

Conversion table: SEER to SEER2, HSPF to HSPF2

These are approximate industry rules of thumb, not exact conversions; the true relationship varies by model. Use the table to sanity-check a comparison, then confirm with the AHRI certificate.

How to compare ratings when you shop

1. Decide which season costs you more

Efficiency gains only save money on energy you actually use. In Houston, cooling dominates and SEER2 drives the bill. In Minneapolis, heating dominates and a jump from 7.5 to 9.5 HSPF2 is worth far more than a jump from 15 to 18 SEER2. Our heat pump cost calculator lets you weigh both.

2. Convert the rating gap to dollars

Savings scale with the ratio of the ratings. Moving from 14.3 to 18 SEER2 cuts cooling electricity by about 20% (1 minus 14.3 divided by 18). If your cooling bill is $600 a year at roughly 18 cents per kWh, that is about $120 a year. Moving from 7.5 to 9.0 HSPF2 cuts heating electricity by about 17%. Multiply by your own bill, then compare that with the price premium and the expected 15-year life of the equipment.

3. Check the cold-weather numbers separately

HSPF2 is a national average that under-weights very cold days. Two units with identical HSPF2 can differ enormously at 5°F: one may keep 100% of its capacity while the other drops to 60% and leans on resistance strips. If your winters see extended stretches below 20°F, look up the model on the NEEP cold-climate list and read the 5°F and 17°F capacity and COP columns. Our guide to the best heat pumps for cold climates does this legwork for the leading models.

4. Match the rating to the exact system

Ratings are certified for a specific outdoor unit paired with a specific indoor coil or air handler. A brand’s headline “up to 24 SEER2” usually applies to one size with one premium indoor unit. Ask your installer for the AHRI reference number for the exact match in your quote. The Lennox SL25XPV review and Carrier Infinity 24 review show how much the certified figure can vary across sizes.

5. Do not overbuy efficiency at the expense of sizing

An oversized 20 SEER2 unit that short-cycles will use more energy and control humidity worse than a correctly sized 16 SEER2 unit. Read our heat pump sizing guide before you settle on an efficiency tier.

Our recommendation

Treat SEER2 as the price of admission and HSPF2 as the number you buy on, unless you live in a cooling-dominated climate. In cold regions, add the COP and capacity at 5°F from the NEEP database to your checklist, because that is what determines whether your heat pump or your backup strips do the work in January. A variable-speed unit in the 16 to 18 SEER2 and 8.5 to 10 HSPF2 range hits the sweet spot for most homes; above that, the premium rarely pays back on energy savings alone. See how the leading models stack up in our best heat pumps of 2026.