The numbers this site uses, and where each one stops describing your house
The short version
- COP is heat delivered divided by energy consumed, dimensionless, and directly comparable to AFUE. A 95% AFUE furnace is a COP of 0.95.
- HSPF2 divided by 3.41214 gives a seasonal COP. HSPF2 11.0 is a COP of 3.22.
- AFUE cannot exceed 100% because a furnace can only release the energy in its fuel. COP can, because a heat pump moves heat rather than making it.
- Balance point is the outdoor temperature where equipment output stops meeting building demand. It belongs to the pairing of machine and house, not to the machine, and it is the number that decides how much expensive backup heat you pay for.
COP, HSPF2, AFUE, balance point and the fuel conversions. Every manufacturer publishes a glossary defining these. What they tend to leave out is the point at which the number stops telling you anything about your own installation, which is the part that decides your bill.
Coefficient of performance
Heat delivered divided by energy consumed, with both in the same units. A machine that delivers 3 kWh of heat while consuming 1 kWh of electricity has a COP of 3.0. Because both sides are in the same units the number is dimensionless, which is what makes it directly comparable to a furnace's AFUE.
A COP above 1.0 is not a trick. A resistance heater cannot exceed 1.0 because every joule of electricity becomes a joule of heat and there is nowhere else for the energy to go. A heat pump is not bound by that because it does not make heat. It runs a refrigeration cycle backwards, absorbing heat from outdoor air and releasing it indoors, and the electricity powers the moving rather than the heat itself.
Where it stops describing your house
COP falls as it gets colder, because there is less heat in the outdoor air and a wider temperature lift to work across. A unit at COP 4.0 at 47F might sit near 2.0 at 5F. Worse, demand rises as COP falls, so the machine is least efficient exactly when you need the most from it.
HSPF2
Heating Seasonal Performance Factor 2, defined by AHRI 210/240-2023. [AHRI 210/240-2023] It is BTU of heat delivered per watt hour of electricity consumed across a heating season. Divide by 3.41214 to get a dimensionless seasonal COP.
| HSPF2 | Seasonal COP | What it indicates |
|---|---|---|
| 8.0 | 2.34 | Builder grade, near the regulatory floor |
| 9.5 | 2.78 | Good mid range |
| 11.0 | 3.22 | Cold climate rated |
| 12.0 | 3.52 | Top of market |
Where it stops describing your house
HSPF2 is measured in one AHRI climate region against an assumed building load line. Your climate is not that climate and your house is not that house. It does account for backup heat, but for the backup the test house needs rather than the backup yours needs.
One trap worth knowing: HSPF2 replaced HSPF in 2023 and tests at a higher, more realistic external static pressure. For the same machine, HSPF2 typically reads 10 to 15% below the old HSPF figure. Older articles and some rebate documents still quote HSPF, and comparing the two makes older equipment look better than it is.
AFUE
Annual Fuel Utilisation Efficiency: the share of fuel energy that becomes useful heat in the house, averaged over a season. Disclosure is required by the FTC Appliance Labeling Rule, [FTC 16 CFR 305] which is why it appears consistently and is one of the more trustworthy numbers in the industry.
It cannot exceed 100%. The installed base clusters in two places: non-condensing appliances around 80%, and condensing appliances at 90 to 98% that recover latent heat by condensing water vapour out of the exhaust. You can usually tell which you have without a service call. White plastic venting through a sidewall with a condensate drain means condensing. Metal flue into a chimney means it is not.
Where it stops describing your house
AFUE is measured at the appliance, on equipment in good condition, correctly sized. It says nothing about ducts. A duct run through an unconditioned attic can lose 20 to 30% of delivered heat, which is a larger effect than the entire difference between an 80% and a 95% furnace. Oversizing and short cycling cost more on top.
Balance point
The outdoor temperature at which a heat pump's output exactly equals the house's heat loss. Above it the heat pump carries the load alone. Below it, backup heat covers the shortfall, and in most installations that backup is electric resistance at a COP of exactly 1.0.
It is a property of the pairing, not of the machine. The same unit has a different balance point in a tight house than in a leaky one. Sizing for the cooling load, which is standard practice in mixed climates, raises it and is the most common cause of a system that leans on its strips.
Why it decides the bill
Because you are summing electricity consumed rather than averaging efficiencies, backup heat combines harmonically and hurts more than its share suggests.
| Share of heat from backup | Effective COP from 3.22 | Efficiency lost |
|---|---|---|
| 0% | 3.22 | 0% |
| 5% | 2.90 | 10% |
| 10% | 2.64 | 18% |
| 15% | 2.42 | 25% |
| 25% | 2.07 | 36% |
| 40% | 1.71 | 47% |
Federal field measurements found auxiliary heat energy exceeding 35% of compressor energy at 5 of 12 monitored homes, and exceeding compressor energy entirely at 2 of them. [NREL/TP-5500-84745 (2023)] The full read is in the evidence note, and the practical version is in why is my heat pump using auxiliary heat.
Fuel conversions
Heating fuels are sold in units that hold different amounts of energy, so comparing sticker prices tells you nothing until you convert.
| Fuel | Retail unit | BTU per unit | kWh per unit |
|---|---|---|---|
| Electricity | kilowatt hour | 3412.14 | 1.00 |
| Natural gas | therm | 100,000 | 29.31 |
| Natural gas | thousand cubic feet | 1,037,000 | 303.91 |
| Propane | gallon | 91,500 | 26.82 |
| Heating oil, No. 2 | gallon | 138,500 | 40.59 |
Converting to energy is only half the job, and skipping the second half is the most common mistake in published comparisons. Divide by the appliance's efficiency to get the cost of heat that actually reaches the room. For a heat pump that efficiency term is greater than one, which is why delivered heat can cost less than the electricity that produced it.
The natural gas figure of 1,037 BTU per cubic foot is the EIA convention. [EIA units and calculators] Real delivered gas runs roughly 1,020 to 1,050 depending on utility and season.
How this was checked
Every constant on this page is imported from the same model file the calculator uses rather than retyped, so the page and the tool cannot drift apart. The therm to kilowatt hour figure was derived rather than quoted and cross-checked against conversion through BTU.
The effective COP function was verified at both limits: zero backup returns the raw COP, full backup returns exactly 1.0.
What this page does not cover
This page defines the terms this site uses. It does not cover SEER2 or cooling performance, refrigerant selection, ground source systems, or the capacity curves of any specific manufacturer's equipment, which should come from published tables rather than from here.
It also cannot calculate your balance point, which needs a heat loss calculation for your building and manufacturer capacity data, neither of which a state level model has.
References
- Air-Conditioning, Heating, and Refrigeration Institute. AHRI Standard 210/240-2023: Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment. This standard defines the M1 test procedure and the HSPF2 metric.
- Mendon, V., Keene, K., Rosenberg, S., Rotondo, J. A., Nwe, K., Young, J., Wind, W., and Goetzler, B. (2025). Performance Results from DOE Cold Climate Heat Pump Challenge Field Validation. PNNL-37127. Pacific Northwest National Laboratory, prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830. January 2025.
- Winkler, J. and Ramaraj, S. (2023). Field Validation of Air-Source Heat Pumps for Cold Climates. NREL/TP-5500-84745. National Renewable Energy Laboratory. May 2023.
- U.S. Federal Trade Commission. Appliance Labeling Rule, 16 CFR Part 305, which governs AFUE disclosure on heating equipment.
- Air Conditioning Contractors of America. Manual J: Residential Load Calculation, the standard method for sizing residential heating and cooling equipment.
- U.S. Energy Information Administration. Units and calculators explained, energy conversion factors and fuel heat content.
- U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. Heat Pump Systems.