SEER2, EER2 and HSPF2 Explained

Three ratings appear on every heat pump and air conditioner. They measure different things, and the one printed largest is often not the one that predicts your bill.

SEER2 — seasonal cooling efficiency

Seasonal Energy Efficiency Ratio, version 2. Cooling output over a whole season divided by electrical energy consumed, across a range of outdoor temperatures.

Higher is more efficient. It's the headline number on the yellow EnergyGuide sticker.

Because it's seasonal, it averages mild days and hot days together. That makes it good for comparing units and mediocre for predicting behavior on your hottest week.

EER2 — efficiency at one hot condition

Energy Efficiency Ratio, version 2. Cooling efficiency at a single, fixed high-temperature test condition — a snapshot, not a season.

EER2 matters where extreme heat persists rather than peaks. In a desert climate with weeks above 100°F, EER2 predicts real performance better than SEER2 does, because that's the condition you'll actually live in.

HSPF2 — seasonal heating efficiency

Heating Seasonal Performance Factor, version 2. Heating output over a season divided by energy consumed, including the energy used by defrost cycles and backup heat.

That last part matters: HSPF2 includes backup heat runtime. A unit with a high balance point that leans on strip heat will show it here.

For a heating-dominated climate, HSPF2 is the number that predicts your bill.

Why "2"

In January 2023, the DOE updated the test procedures. The new tests use higher external static pressure, which better reflects a real ducted system rather than a laboratory bench with minimal restriction.

Practical consequences:

• SEER2 ≈ 4–5% lower than the old SEER for identical equipment
• HSPF2 ≈ 8–10% lower than the old HSPF
• Minimum standards vary by region for cooling — the South and Southwest have higher cooling minimums than the North

The equipment didn't get worse. The test got more honest.

Don't compare a SEER number to a SEER2 number. Old listings, old marketing, and older units all quote the pre-2023 figures, and a "16 SEER" unit is roughly equivalent to a "15.2 SEER2" unit.

What the ratings don't tell you

This is the important part for heat pumps.

Seasonal ratings hide low-temperature capacity. HSPF2 is an average across a season. Two units with the same HSPF2 can behave very differently at 5°F — one maintaining 85% of rated capacity, the other 40%.

That difference determines your balance point, how much backup heat runs, and whether you need a service upgrade. And it doesn't appear in HSPF2 at all.

Ask for the capacity table — rated output at 47°F, 17°F, and 5°F. That's the specification that matters in a cold climate. See cold climate heat pumps.

How does COP relate to HSPF2?

You'll see both, and they measure the same thing in different units.

COP (coefficient of performance) is dimensionless — units of heat out per unit of energy in. HSPF2 is in BTU per watt-hour. Since 1 watt-hour is 3.412 BTU, converting is one division:

COP = HSPF2 ÷ 3.412

So an HSPF2 of 8.5 is a seasonal average COP of about 2.5 — two and a half units of heat per unit of electricity, averaged across the whole heating season. The same conversion works for cooling: EER2 ÷ 3.412 gives the cooling COP.

The distinction that matters: a manufacturer quoting "COP 3.2" is usually stating an instantaneous figure at a specific outdoor temperature, while HSPF2 is a seasonal average including defrost cycles and backup heat. They're not comparable numbers, and the instantaneous one is always the flattering one.

When you ask for low-temperature performance, ask for COP at a stated temperature — that's the number that tells you what happens at 5°F.

How do you turn a rating into dollars?

The ratings are BTU per watt-hour, so dividing your seasonal load by the rating gives kilowatt-hours directly.

Annual kWh = seasonal BTU load ÷ rating

Say your heating season needs about 30 million BTU:

• At HSPF2 8.5: 30,000,000 ÷ 8.5 = 3,529 kWh → about $600 at $0.17/kWh
• At HSPF2 10.0: 30,000,000 ÷ 10 = 3,000 kWh → about $510

So the efficiency jump saves roughly $90 a year in that example. Set that against the price difference between the two units and you have your answer: a $500 premium pays back in about five and a half years, a $2,000 premium doesn't pay back within the equipment's life.

The same arithmetic works for cooling with SEER2. What changes the conclusion far more than the rating does is the size of the seasonal load — which is why the same premium is obviously worth it in one climate and obviously not in another.

Run your own numbers in the heat pump payback calculator.

Where do the official numbers come from?

The published ratings live in the AHRI Directory of Certified Product Performance, and there's a detail there that catches a lot of buyers.

Ratings apply to a matched combination, not to the outdoor unit by itself. Each certified pairing of outdoor unit, indoor coil or air handler, and sometimes a specific control has its own AHRI Reference Number and its own rated SEER2, EER2 and HSPF2.

Two consequences:

• A mismatched system doesn't achieve the advertised rating. Pairing a high-efficiency outdoor unit with an old indoor coil is common in "we'll just replace the outside unit" quotes, and the resulting system performs nowhere near the number on the outdoor unit's box.
• Rebate and incentive programs routinely ask for the AHRI reference number, precisely because it identifies the actual combination rather than a component.

Ask your contractor for the AHRI reference number of the exact combination they're quoting, and look it up. It takes two minutes and it's the only way to confirm the numbers in the proposal describe the system you're actually buying.

One note for ductless: mini splits are rated under the same procedure, but non-ducted units are tested without the ductwork static pressure that ducted systems face. That's part of why ductless SEER2 figures read high, and it's a genuine advantage rather than a testing artifact — there really are no duct losses.

Is higher worth paying for?

Efficiency costs money upfront and saves it over time. Whether that trade works depends entirely on how many hours the equipment runs.

Rough shape of it:

The arithmetic: estimate annual runtime hours, compute the energy difference between the two efficiency levels at your electricity rate, and compare against the price difference. In a mild climate, the answer is frequently that the mid-tier unit is the better buy.

Run it with the heat pump payback calculator and current rates from the electricity cost calculator.

Installation quality outrans the rating

A 20 SEER2 system with a poor refrigerant charge, restricted ductwork, or bad airflow will underperform a well-installed 15 SEER2 system.

The ratings are laboratory numbers under controlled conditions. Real performance depends on correct sizing, ductwork that can carry the airflow, an accurate charge, and commissioning that verifies both.

Which means: money spent on a good installation of mid-tier equipment usually returns more than money spent on premium equipment installed carelessly. See heat pump installation cost and oversized HVAC problems.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• DOE 10 CFR Part 430 — the 2023 test procedure update that introduced SEER2, EER2 and HSPF2
• AHRI 210/240 — the rating standard for unitary equipment

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• How to choose a heat pump
• Cold climate heat pumps
• Variable speed vs single stage heat pumps

More in our heating and cooling guides.