A heat pump’s efficiency is the whole story of its operating cost, and it is captured by two numbers: the instantaneous COP at a given condition and the seasonal HSPF across a whole winter. This tool computes the exact COP from your output and power readings and estimates the seasonal HSPF from the two AHRI rating points, so you can evaluate performance in the field and compare equipment on paper.
How it works
The instantaneous COP is a direct unit conversion, and the HSPF is a bin-hour sum:
COP = BTU/h output / (watts × 3.412) (1 W = 3.412 BTU/h)
HSPF = Σ(BTU delivered per bin) / Σ(Wh consumed per bin)For HSPF the tool interpolates the COP at each outdoor-temperature bin between your rated COP at 17 °F and 47 °F, models a building load that rises as it gets colder, and sums the heat delivered and electricity consumed across the DOE Region IV (Atlanta) standard bin-hour distribution. Dividing seasonal BTU output by seasonal watt-hours gives the HSPF in its native BTU-per-watt-hour units.
Why COP and HSPF tell different stories
COP answers the question “how efficiently is this unit running right now?” It changes with outdoor temperature, indoor set-point, defrost cycles, and system static pressure. A heat pump may achieve a COP of 4.0 on a mild 45 °F day but only 2.0 on a frigid 15 °F night. Taking a single COP reading in comfortable conditions overstates seasonal efficiency.
HSPF answers “how much heat per watt-hour do I get across an entire heating season?” It is the number that appears on the yellow EnergyGuide label and in AHRI certification data. A unit with an HSPF of 9 delivers 9 BTU of heat for every watt-hour of electricity it consumes over the season — roughly equivalent to a seasonal average COP of about 2.6.
Worked example
A technician measures a heat pump on a 40 °F morning:
- Heating output: 42,000 BTU/h
- Power draw: 3,600 watts
COP = 42,000 ÷ (3,600 × 3.412) = 42,000 ÷ 12,283 ≈ 3.42
That is a healthy result at that outdoor temperature. To estimate the seasonal performance, the technician also notes the AHRI-rated COPs: 3.8 at 47 °F and 2.4 at 17 °F. Entering those into the HSPF estimator produces an approximate HSPF in the range of 8 to 9, consistent with the equipment’s published EnergyGuide label.
Reading COP in context
| COP value | Meaning |
|---|---|
| 1.0 | Equivalent to electric resistance heat |
| 2.0–2.5 | Typical at very cold outdoor temperatures |
| 3.0–3.5 | Good efficiency at mild-to-cold conditions |
| 4.0–5.0 | Excellent — achievable on mild days or with inverter units |
A unit that consistently measures COP below 2.0 at outdoor temperatures above 25 °F may have a refrigerant charge problem, dirty coils, or a failing compressor. COP measurement is a useful diagnostic alongside duct leakage checks and indoor delta-T tests.
HSPF2 versus HSPF
Starting in 2023, the US DOE shifted to an updated metric called HSPF2, which uses a more realistic system conditions test (higher static pressure, adjusted bin temperatures). HSPF2 values are numerically lower than HSPF for the same unit — a unit rated HSPF 10 may be rated HSPF2 8.1. The estimator in this tool follows the classic HSPF bin-hour method for compatibility with pre-2023 rating data; verify which metric a manufacturer is quoting before comparing models across the changeover date.