Four ratings decide how cheap a heating or cooling system is to run. They sound like alphabet soup, but each one plugs straight into the running-cost math. Here’s the plain-English version.
AFUE — furnace efficiency
AFUE (Annual Fuel Utilization Efficiency) is the share of a furnace’s or boiler’s fuel that becomes useful heat. A 95% condensing gas furnace turns 95 cents of every fuel dollar into heat; an older 80% unit wastes a fifth up the flue. In the cost formula, fuel used = heat demand ÷ AFUE. More on AFUE.
HSPF / HSPF2 — heat-pump heating efficiency
HSPF (Heating Seasonal Performance Factor) rates a heat pump’s seasonal heating efficiency in BTU per watt-hour. HSPF2 is the stricter 2023 test (about 15% lower numbers for the same unit). Divide HSPF by 3.412 to get a seasonal COP. More on HSPF.
COP — heat out per energy in
COP (coefficient of performance) is the headline number for running cost: heat delivered per unit of electricity. A COP of 3 means 3 kWh of heat per 1 kWh of power — 300% efficient — versus 1.0 (100%) for electric resistance. COP falls in cold weather, which is why we use climate-zone presets. More on COP.
SEER / SEER2 — cooling efficiency
SEER (Seasonal Energy Efficiency Ratio) rates cooling; SEER2 is the 2023 standard. Because a heat pump also air-conditions, SEER2 sets your summer bill the way COP/HSPF set the winter one. More on SEER.
How they fit together
| Rating | Applies to | Drives | Higher is… |
|---|---|---|---|
| AFUE | Furnaces, boilers | Fuel use in winter | Better |
| HSPF2 | Heat-pump heating | Electricity use in winter | Better |
| COP | Heat-pump heating (instant/seasonal) | Electricity use in winter | Better |
| SEER2 | Heat-pump / AC cooling | Electricity use in summer | Better |
When you compare a heat pump against gas, propane or oil, AFUE and COP/HSPF are the levers that move the result. Plug your own numbers into the calculator and watch how a higher COP or AFUE shifts the winner.