Method · updated 2026-10-26
How we verify every number
The short answer
Every figure comes from a public data file we downloaded and dated, run through one published formula. We never type a price or an efficiency by hand. Weather is NOAA, prices are EIA, equipment is ENERGY STAR (EPA), geography is Census. Where a source was unavailable we say so below instead of filling the gap.
Inputs
| Input | Source | Notes |
|---|---|---|
| Hourly temperature | NOAA ISD-Lite via the AWS open-data bucket | 122 airport stations, 2015–2024. Lowest coverage: Minneapolis, MN at 87.5% of hours. |
| Electricity price | EIA electricity data | Residential, trailing twelve months to August 2026, per state. |
| Gas, oil, propane | EIA SEDS | 2024 state residential prices per million BTU of fuel. Annual average, so seasonal swings are not modelled. |
| Heat pump capacity and efficiency | ENERGY STAR Certified Mini-Split Heat Pumps | 14,941 cold-climate rows from 218 brands: capacity at 47°F, 17°F and 5°F, COP at 5°F, HSPF2. |
| ZIP to climate | Census 2020 ZCTA | ZIP centroid to the nearest of our 122 stations by great-circle distance, and to the state with the largest county overlap. |
Formulas
- Design temperature is the 99% hourly percentile of 2015–2024 temperatures at the station (the temperature that 99% of hours are warmer than). It is not the ASHRAE published value, which is licensed; ours comes from a ten-year window and may be milder.
- Load line. Heat loss is zero at 65°F and equals your design load at the design temperature, straight line in between. UA = load ÷ (65°F − design temp).
- Capacity. Linear between the 5°F and 47°F ENERGY STAR points, flat above 47°F, never rising below 5°F. The 17°F number is displayed but not used, because it is reported under a different test condition.
- COP. Linear from the 5°F COP to the seasonal COP (HSPF2 ÷ 3.412) at 47°F. This is conservative because HSPF2 already averages cold hours.
- Backup. Whenever load exceeds capacity, the shortfall is met by electric resistance at 1.0 COP.
- Cost. Σ over 5°F bins of (hours × energy) × price. Fuels divide by the furnace or boiler efficiency (90% unless you change it in the calculator).
- Reference unit. The median across all 14,941 cold-climate systems: 5°F capacity 88% of 47°F (10th to 90th percentile: 71% to 103%), HSPF2 10, COP at 5°F 2. Unit sized so rated capacity at the design temperature equals the load.
What we do not model
- Internal gains, solar gain, defrost cycles, duct or line-set losses, humidity and thermostat setbacks.
- Cooling, or electricity rates that vary by time of day or tier.
- Your house: use a Manual J. All our load scenarios (12,000 / 24,000 / 36,000 / 48,000 BTU/h) are illustrations we chose, not data.
What we could not verify
- NEEP's cold-climate list. No bulk download is offered, so we use ENERGY STAR's own cold-climate flag instead. They are not identical lists.
- ASHRAE design temperatures, licensed, so replaced by our own percentiles.
- Installed and kit prices, and price history: no public bulk source. The price tracker on this site is intentionally empty.
- State and utility rebates: DSIRE's API refused access; DOE rebate pages returned 404.
Changes to this method are dated here. Last reviewed 2026-10-26. Raw and cleaned files are on the data page.