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

Heat pump SEER2 measures seasonal cooling efficiency, HSPF2 measures seasonal heating efficiency, and COP measures heating efficiency at a single outdoor temperature. Higher is better for all three. In Seattle, HSPF2 matters most, because heating does most of the work, and City Light’s discounts require both SEER2 and HSPF2 to clear set thresholds.

Efficiency ratings are where heat pump quotes most often talk past each other. The Seattle Heat Pump Company plans and manages heat pump replacements and ducted installations in Seattle and King County, with the work carried out by licensed and insured contractors, and every system we plan is chosen against a matched-system rating. This guide explains what those numbers mean and how they connect to your bill and your rebate.

The three numbers at a glance

SEER2: cooling efficiency

SEER2 stands for seasonal energy efficiency ratio, measured under the federal test procedure that took effect January 1, 2023. It divides the cooling a system delivers over a modeled cooling season, in BTU, by the electricity it uses, in watt-hours. A SEER2 of 16 means 16 BTU of cooling for every watt-hour.

The “2” matters. The newer test pushes air through the indoor unit against higher external static pressure, which better represents the resistance real ductwork adds. The same equipment rates lower on SEER2 than it did on the old SEER scale, so a SEER2 of 15.2 is not a step down from an old SEER of 16. Compare like with like.

For a Seattle house, SEER2 is a secondary number but not a trivial one. Sea-Tac’s cooling degree days roughly doubled between the 1990s and the last decade, so summer efficiency is worth more than it used to be. EER2, which rates efficiency at a single hot condition, tells you more about performance during a heat wave like June 2021’s.

HSPF2: heating seasonal efficiency

HSPF2 is the heating seasonal performance factor. It divides the heat a system delivers over a modeled heating season by the electricity it uses, including the energy spent on defrost cycles and on electric backup heat in the coldest modeled hours. Like SEER2, it uses the 2023 test procedure and runs lower than the old HSPF scale for the same equipment.

HSPF2 and COP are directly related. Because one watt-hour equals about 3.412 BTU, dividing HSPF2 by 3.412 gives a rough seasonal average COP under the test’s assumptions. An HSPF2 of 8.1 works out to a seasonal COP of about 2.4; an HSPF2 of 9.5, about 2.8.

That relationship is useful, with one caution. The HSPF2 test models a reference climate defined by the federal procedure, not Seattle’s. Seattle’s winter is long and mild, with only about one night a year at 20°F or colder at Sea-Tac, so a heat pump here spends most of its time in conditions where it runs efficiently. A system’s real seasonal COP in Seattle can differ from the HSPF2 estimate in either direction, depending on how it was installed and controlled.

COP: heating efficiency at one moment

The coefficient of performance is the simplest of the three: heat delivered divided by the energy used to deliver it, at one set of conditions. A COP of 3 means three units of heat for one unit of electricity. Electric baseboard and the backup heat strips in an air handler have a COP of 1.

A heat pump’s COP is not fixed. It is highest in mild weather and falls as the outdoor temperature drops, because there is less heat in colder air to move. That is why manufacturers publish COP at standard test temperatures, commonly 47°F and 17°F, and for cold-climate models at 5°F. ENERGY STAR’s cold-climate designation sets a minimum COP at 5°F.

COP is the number that connects efficiency to fuel cost. On Seattle City Light’s current rate of 13.38 cents per kWh, a heat pump costs less to run than a 95% efficient gas furnace whenever its COP is above about 2.2. On City Light’s suburban rate the break-even is about 2.4, and on PSE electricity about 3.4. The gas furnace comparison and the operating cost guide work through that math.

Federal minimums

Since January 1, 2023, new split-system heat pumps sold in the United States must meet at least 14.3 SEER2 and 7.5 HSPF2. Anything quoted below that is old stock or a mistake. City Light’s lowest discount tier starts well above the federal floor, at SEER2 15.2 and HSPF2 8.1, which is one reason the minimum-efficiency unit is rarely the best value in Seattle.

AHRI matched ratings: why the outdoor unit is not enough

A ducted heat pump’s efficiency belongs to the combination, not to the outdoor unit alone. The indoor coil, the air handler or furnace blower, and sometimes the expansion device all affect how much heat moves per watt-hour. The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) certifies ratings for specific matched combinations, and each gets a reference number in AHRI’s directory.

That has three practical consequences:

• The same outdoor unit can carry different SEER2 and HSPF2 ratings depending on the indoor equipment it is paired with.
• A quote that lists only the outdoor model cannot be checked against a rebate tier; the quote comparison guide covers what else a quote should list.
• A replacement that reuses an old indoor coil with a new outdoor unit may have no certified rating at all, which is one reason most replacements now change both. Washington’s refrigerant rule makes that even more common; see the refrigerant change guide.

Ask for the AHRI certificate or reference number for the exact combination being quoted. It is the cleanest way to confirm which rebate tier a system meets.

City Light tier thresholds

City Light’s contractor discount is tied to the matched system’s SEER2 and HSPF2. Both numbers must meet the tier’s minimums.

A system at SEER2 17 and HSPF2 9.0 qualifies for Tier 1, not Tier 3, because its HSPF2 falls short. For a Seattle house, that is the right emphasis: HSPF2 is the harder threshold and the one that matters more for the bill. The discount is limited to one system per home, paid through participating distributors and contractors, and City Light strongly recommends passing it to the customer. The rebate guide covers the other conditions.

Why ratings do not equal your bill

A rating is a lab result for a properly installed system. Your bill depends on how close the real installation comes to that.

• Airflow. Restrictive ducts reduce airflow across the indoor coil and lower real efficiency. A static pressure test shows whether your ducts are a limit; see using a heat pump with existing ductwork.
• Refrigerant charge. Over- or undercharging cuts efficiency and capacity. Commissioning should verify charge against the manufacturer’s method; the installation process guide describes what that involves.
• Sizing. An oversized system cycles; an undersized one leans on backup heat with a COP of 1. See the sizing guide.
• Controls. A thermostat that calls for backup heat too readily, or a dual fuel changeover set too high, can erase the efficiency you paid for.
• Your rate. The same HSPF2 saves more money on City Light’s rate than the break-even math suggests on PSE’s.

Efficiency ratings tell you the ceiling. Design, installation and setup decide how close you get to it.

Reading a heat pump spec sheet

Beyond SEER2 and HSPF2, these figures on a manufacturer’s data sheet are worth a look for a Seattle house:

• heating capacity at 47°F and 17°F, which shows how much capacity the unit keeps as it gets colder
• COP at 17°F, and at 5°F for cold-climate models
• the minimum and maximum capacity of a variable-speed unit, which shows how far it can turn down in mild weather
• the outdoor unit’s sound rating in dBA, which feeds Seattle’s noise review
• the refrigerant, which must have a global warming potential under 750 for new equipment in Washington since 2026

Variable-speed equipment generally sits higher on all of these; the variable-speed comparison explains why, and the Seattle winter guide explains how low-temperature capacity fits Seattle’s climate.

Next step

The best rating for your house is the one that matches its load, its ducts and your budget, and clears the rebate tier that makes financial sense. The replacement cost guide shows how efficiency tier moves the price, and the cost estimator compares tiers for your house. For a recommendation built on a load calculation, call or send a few details about the system you have now.