Enter heat pump and solar assumptions
Example data table
| Thermal Capacity | COP | Hours/Day | PSH | Panel (W) | PV Needed (kW) | Panels |
|---|---|---|---|---|---|---|
| 8.0 kW | 3.2 | 6 | 4.5 | 550 | ~3.97 | 8 |
| 27,000 BTU/h | 3.5 | 5 | 5.0 | 450 | ~2.23 | 5 |
| 12.0 kW | 2.8 | 7 | 4.0 | 600 | ~7.06 | 12 |
Formula used
- Thermal to electric input: Electric kW = Thermal kW ÷ COP
- Daily energy: Daily kWh = Electric kW × Hours/day
- Net solar yield per kW: Net = PSH × Derate × Inverter × Temp
- PV size: PV kW = Daily kWh ÷ Net
- Panels: Panels = ceil(PV kW × 1000 ÷ Panel W)
- Backup battery: Battery kWh = (Electric kW × Backup hours) ÷ (DoD × Efficiency)
How to use this calculator
- Pick the capacity unit, then enter your heat pump heating capacity.
- Enter an average COP that matches your season and equipment.
- Use realistic runtime hours and days for the heating period.
- Set local peak sun hours and adjust loss factors conservatively.
- Choose panel wattage; the tool rounds up to whole panels.
- Optional: enter backup hours to estimate battery capacity.
- Press Calculate to see results above the form.
Link heat delivery to electricity
Heat pump demand is driven by delivered heat and efficiency. Converting thermal capacity to electrical input uses Electric kW = Thermal kW ÷ COP. For example, an 8 kW unit at COP 3.2 draws about 2.50 kW while running. If runtime averages typical 6 hours per day, energy use is about 15.0 kWh, before adding fans, defrost cycles, or auxiliary resistance heat.
Translate sunlight into usable energy
Solar production depends on peak sun hours and real‑world losses. Net yield per installed kW is PSH × derate × inverter efficiency × temperature factor. With PSH 4.5, derate 0.78, inverter 0.96, and temp 0.95, one installed kW produces about 3.20 kWh per day. Winter PSH can be lower, so conservative inputs reduce the risk of undersizing during the heating season.
Convert daily energy into panel capacity
PV sizing compares daily load to net daily yield. PV kW = Daily kWh ÷ Net yield. Using 15.0 kWh/day and 3.20 kWh/kW/day suggests 4.69 kW of panels to cover that day, before rounding to whole modules. Panel wattage sets the count; 550 W modules round this example to 9 panels (4.95 kW). In practice, roof layout, shading, and orientation may push the rounded array higher.
Estimate monthly value and seasonal risk
Monthly results scale by days in the heating period and help estimate savings. With 30 days, the example load is 450 kWh per month. A 4.95 kW array at 3.20 kWh/kW/day produces about 475 kWh, so the offset is capped near the load unless net export is credited. Costs use your tariff; at $0.16/kWh, that is roughly $72 per month in avoided energy charges, or about $864 annually if conditions stay similar.
Optional backup and emissions context
Backup sizing is optional but useful for outages and load shifting. Battery capacity is (Electric kW × backup hours) ÷ (DoD × battery efficiency). For 2 hours of backup, DoD 0.80, and efficiency 0.92, the required capacity is about 6.8 kWh. Emissions avoided can be estimated as offset kWh × grid factor, such as 0.45 kg CO2/kWh, which yields roughly 203 kg CO2 avoided per month in the example.
What does COP mean for sizing?
COP links heat delivered to electricity used. Higher COP lowers input kW and reduces required solar size. Use a seasonal or average COP that matches your climate, equipment, and operating temperatures.
Should I size for winter or annual sun hours?
If your goal is heating coverage, use winter or conservative peak sun hours. Annual averages can oversize savings but understate winter needs. Many designs balance winter performance with annual net metering rules.
Why do derate and temperature factors matter?
Panels rarely produce nameplate output. Dirt, wiring, mismatch, aging, inverter losses, and heat reduce yield. Derate and temperature factors convert ideal sun hours into realistic energy, improving sizing accuracy.
How is panel count calculated?
The calculator converts required PV kW into modules using Panels = ceil(PV kW × 1000 ÷ panel wattage). Rounding up ensures the array meets or slightly exceeds the target energy under your assumptions.
Does this include starting surge or defrost loads?
It estimates energy from average electrical input. Short surges, defrost cycles, and auxiliary heaters can raise peak power. Consider adding margin or using a larger inverter if your equipment has frequent high‑power events.
How do I use the battery backup option?
Enter the backup hours you want the heat pump to run. The tool computes battery kWh using DoD and efficiency. Treat the result as a minimum; add capacity for other loads, cold‑weather losses, and battery aging.
Assumptions and tips
- Peak sun hours vary by month; use a conservative value for winter heating.
- Derate includes soiling, wiring, mismatch, and long-term degradation.
- Temperature factor approximates output changes; site conditions matter.
- For net metering, annual offset can differ from monthly timing.