Sizing basics
SEER2, HSPF2, AFUE and COP: the ratings explained
Every efficiency rating is output divided by input. Once you know what each one counts, comparing equipment and estimating what it costs to run is simple arithmetic.
One idea behind every rating
Each efficiency rating on heating and cooling equipment is the useful output divided by the energy input, measured under a standard test so that different products can be compared. They differ in what they count as output, what they count as input, and the conditions of the test. The higher the rating, the less energy the equipment uses for the same heating or cooling, and because energy use is proportional to one over the rating, the arithmetic of savings is the same for all of them.
The ratings at a glance
| Rating | Equipment | What it measures | Typical new range |
|---|---|---|---|
| SEER2 | Air conditioners, heat pumps (cooling) | Seasonal cooling in BTU per watt-hour of electricity | 13.4 to 22+ |
| EER2 | Air conditioners, heat pumps (cooling) | Cooling in BTU per watt-hour at 95 °F outdoors | 10 to 14 |
| HSPF2 | Heat pumps (heating) | Seasonal heating in BTU per watt-hour | 7.5 to 10+ |
| COP | Heat pumps, any heater | Heat delivered ÷ energy used, both in the same units | 2 to 4.5 (heat pumps) |
| AFUE | Gas, propane and oil furnaces and boilers | Annual heat delivered ÷ fuel energy burned | 80% to 98.5% |
| UEF | Water heaters | Hot-water energy delivered ÷ energy used over a test day | 0.6 to 4 |
The full table, with the federal minimums and conversions, is on the HVAC efficiency ratings table.
What changed in 2023
In January 2023 the Department of Energy changed the test procedure for central air conditioners and heat pumps. The new test, called M1, uses a higher external static pressure, 0.5 inches of water column instead of 0.1, which is much closer to what real ductwork imposes on the blower. The same equipment therefore uses more fan energy in the test and earns a lower number, and the ratings were renamed SEER2, EER2 and HSPF2 to make the change visible.
The conversions are roughly SEER2 = SEER × 0.95 and HSPF2 = HSPF × 0.85. A unit sold before 2023 as SEER 16 is about SEER2 15.2. This matters whenever you compare an old unit with a new one: the SEER upgrade savings calculator asks which scale your old label uses and converts it, because comparing SEER 10 with SEER2 15.2 directly would overstate the saving by 5%.
The federal minimums since 2023 are 13.4 SEER2 for air conditioners in the northern states and 14.3 SEER2 in the southeastern and southwestern regions, with EER2 minimums in the hot-dry Southwest; heat pumps must meet 14.3 SEER2 and 7.5 HSPF2 nationwide.
From a rating to a running cost
Because a rating is output over input, dividing the output by the rating gives the input. For cooling:
A 3-ton unit at SEER2 15 draws 36,000 ÷ 15,000 = 2.4 kW. Running 8 hours a day for 30 days at 17 cents per kWh, that is 576 kWh and $97.92, which is what the AC running cost calculator reports. On a 95 °F afternoon the unit runs nearer its EER2, about 12% lower than its SEER2, so a heat wave costs more per hour than the seasonal average.
For a heat pump, the seasonal coefficient of performance is HSPF2 divided by 3.412, so HSPF2 8.5 is a COP of 2.49; a heat pump delivering 30,000 BTU per hour at that COP draws 3.53 kW. For a furnace, the gas burned is the output divided by AFUE. The HVAC running cost calculator handles all of these with the same arithmetic.
From two ratings to a saving
The saving from a higher rating is the old energy use times one minus the ratio of the ratings:
Two upgrades
An old SEER 10 air conditioner (SEER2 9.5) replaced with a SEER2 15.2 unit, 3 tons, 1,200 full-load hours, 17 cents per kWh:
- Old: 43.2 million BTU ÷ 9,500 = 4,547 kWh, $773 a year
- New: 43.2 million BTU ÷ 15,200 = 2,842 kWh, $483 a year
- Saving: $289.89 a year, 38% less electricity
An 80% furnace replaced with a 96% one on a $1,400 heating bill:
- New cost: $1,400 × 80 ÷ 96 = $1,167
- Saving: $233 a year, 17%
The SEER upgrade savings calculator does the first from tonnage and hours; the HVAC upgrade payback calculator does the second for any rating from your bill, and adds the payback against the extra cost.
Diminishing returns
Each step up in rating saves less than the one before, because the saving depends on one over the rating. Going from SEER2 9.5 to 15.2 cuts cooling electricity by 38%; going from 15.2 to 18 cuts it by a further 16% of the new figure. The same is true of AFUE, where 80% to 96% saves 17% and 96% to 98% saves only 2%. Higher tiers often come with variable-speed compressors and blowers that run quieter and remove more moisture, which can be worth more than the bill saving.
Comparing different fuels
Ratings compare equipment of the same kind. To compare a heat pump with a gas furnace, the rating has to be combined with the price of the energy, which is what cost per million BTU does: gas at $1.50 per therm in a 95% furnace is $15.79 per million BTU delivered, a heat pump at COP 2.8 and 17 cents per kWh is $17.79. The fuel cost per million BTU calculator ranks every fuel, and the guide to comparing fuels per million BTU explains the method. For heat pumps in cold weather, the guide to balance points and backup heat explains why the seasonal COP falls in winter.
What the ratings do not tell you
A rating is measured on a test stand with ideal installation. In a house, leaky ducts, a wrong refrigerant charge, a dirty coil, an oversized unit and poor airflow can each take 10 to 30% off. A high-SEER2 unit installed badly can perform worse than a mid-tier unit installed well, which is why the guide to what HVAC installations cost stresses installation quality as much as the tier.