How Many Solar Panels for a Heat Pump?
Built by Jeremy Panasuk. Published October 6, 2026.
The panel count starts from a published annual kilowatt-hour figure, split evenly across 365 days, then run through this site's array formula. That even split is a bridge into the calculator. It is not a winter design day.
sizeSystem does 30,109.589 ÷ 4 ÷ 0.86 = 8,752.8 W and rounds up to 8,800 W. Enter a daily load you actually measured in the solar size calculator.What the DOE tables are
FEMP's acquisition guidance applies to single-package and split-system residential air-source heat pumps that operate on single-phase current and have cooling capacities less than 65,000 Btu/h. Models on three-phase current are excluded. Annual energy use in each table is based on the test method in 10 CFR 430 Subpart B Appendix M, for a 36,000 Btu/h unit, using hour counts from the 2016 Technical Support Document for residential central air conditioners and heat pumps.
The ENERGY STAR column is SEER2/HSPF2 of 15.2/7.8 in all three tables. Annual heating-and-cooling energy in that column is:
| DOE table | ENERGY STAR annual kWh | Hours per year in the test method |
|---|---|---|
| Hot-humid / Southeast | 11,660 kWh | 3,259 cooling and 854 heating |
| Hot-dry / Southwest | 10,743 kWh | 3,285 cooling and 642 heating |
| Northern | 10,990 kWh | 2,222 cooling and 1,241 heating |
Northern states, in DOE's note, are all states not included in the hot-humid and hot-dry lists. The hot-humid annual figure is higher than the northern one. The northern heating-hour count is the longest of the three. This page uses 10,990 kWh because an off-grid array is often outside the hot-humid and hot-dry sets. It is not a national average.
The room-by-room heating and cooling load is a different calculation. That belongs on the HVAC load calculator, not in these annual test-method hours.
One year, split into a calculator day
10,990 kWh × 1,000 = 10,990,000 Wh per year. Divided by 365, that is 30,109.589 Wh per day. The division treats every day as equal. DOE's northern heating mode is 1,241 hours per year, which is not 24 hours of heating every day, and the cooling mode is 2,222 hours per year. The daily watt-hour figure is this page's input to the calculator. It is not the energy of the coldest day.
sizeSystem in assets/app.js sets array watts to daily watt-hours ÷ sun hours ÷ 0.86, then rounds up to the next 100 W. The 0.86 is 1 − 0.14, Table 6 of the NREL PVWatts Version 5 manual (default system losses: soiling, shading, snow, mismatch, wiring, connections, light-induced degradation, nameplate, age, and availability). It excludes inverter and battery round-trip losses.
| Sun hours typed in | Division | Rounded array |
|---|---|---|
| 4 | 30,109.589 ÷ 4 ÷ 0.86 = 8,752.8 W | 8,800 W |
| 2.5 | 30,109.589 ÷ 2.5 ÷ 0.86 = 14,004.5 W | 14,100 W |
At the form's default of 2 days and 80% depth of discharge, bank watt-hours are 30,109.589 × 2 ÷ 0.8 = 75,274.0 Wh. At 48 V that is 75,274.0 ÷ 48 = 1,568.2 Ah, which the 10 Ah step rounds up to 1,570 Ah. sizeController on the 8,800 W array at 48 V is 8,800 ÷ 48 ÷ 0.85 = 215.7 A. The listed controller sizes stop at 100 A, so the function's fallback, Math.ceil(amps / 20) × 20, is 220 A. That 220 A is the function's fallback, not a controller model.
Cabin context for why heat is usually kept off the array is on off-grid cabin solar. The controller-amp function itself is on how to size a solar charge controller.
Resistance heat is a separate load
DOE says residential heat pumps are frequently selected on cooling capacity, and electric resistance elements are typically added to make up for heating-capacity deficiencies. Because those elements are inefficient, DOE says their use should be kept to a minimum, and that thermostats designed for heat pumps should ramp temperature slowly so they do not activate the resistance elements. DOE also says cold climates require specially designed heat pumps that can operate at lower ambient temperatures before switching to resistance heating.
The 10,990 kWh figure is the table's annual heating-and-cooling energy for the test-method hours. This page does not claim that number already includes a particular house's resistance-strip hours, and it does not claim the number excludes them. If strip heat runs, its watt-hours are an additional daily load you measure and type into the calculator. They are not inside the 8,800 W line unless you add them.
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One common mistake
Treating annual kilowatt-hours ÷ 365 as the design-day load. The northern heating-hour count is 1,241 hours per year. A cold day can draw far more than 30,109.589 Wh, and a mild day far less. The other mistake is using the Southeast 11,660 kWh, or the Southwest 10,743 kWh, as if it were the northern year.
Limits
DOE says federal buyers should require installation in accordance with ANSI/ACCA 2015, HVAC Quality Installation Specification. Installation problems DOE names include oversizing, improper charging, and leaky ducts. The electrical service for a heat pump is a licensed electrician's job. This page does not size a breaker, a wire, or a house. The 8,800 W and 14,100 W lines are the calculator's rounding of one even split. Run your own daily watt-hours, and your own worst-month sun hours, before buying panels.
FAQ
Is 8,800 W the winter array?
No. 8,800 W is sizeSystem at 4 sun hours on an even split of the northern 10,990 kWh year. The same daily watt-hours at 2.5 sun hours round up to 14,100 W. Neither figure is a design-day load. The northern test method uses 1,241 heating hours per year, not 24 hours a day.
Do the three DOE regions share one annual kWh figure?
No. The ENERGY STAR column for a 36,000 Btu/h unit is 11,660 kWh in the hot-humid/Southeast table, 10,743 kWh in the hot-dry/Southwest table, and 10,990 kWh in the northern table. This page runs only the northern figure through the calculator.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- U.S. Department of Energy, FEMP, Purchasing Energy-Efficient Residential Air-Source Heat Pumps — guidance updated December 2024. ENERGY STAR annual kWh, SEER2/HSPF2 15.2/7.8, test-method hours, scope under 65,000 Btu/h, resistance-heat tip, cold-climate sentence, and ANSI/ACCA 2015 installation. Accessed Oct 6, 2026.
- NREL PVWatts Version 5 Manual, A. Dobos, September 4, 2014 — Table 6, 14% default system losses; 1 − 0.14 = 0.86. Accessed Oct 6, 2026.
- SolarSizer calculator, assets/app.js —
sizeSystemandsizeController. Accessed Oct 6, 2026.