Heat pump with solar calculator
Computes the overlap of PV yield and heat pump electricity hour by hour — not from an estimated daytime share. For 10 kWp and 14,000 kWh of heat near Berlin, 25 % of the heat pump electricity falls in hours with your own PV yield: 19 % in winter, 49 % in summer. That opposition is the answer, and it is soberer than brochures suggest.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
Data as of: 2026-07-30
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Heating degree days of the location | HDD_m = max(0, T_base - T_mean,m) * d_m | 1,752.8 Kd | measured |
| Heat pump electricity demand | W_el,m = (Q_heat,m + Q_DHW,m) / SCOP | 4,000 kWh | assumed |
| Direct overlap | cover_m = min(PV_m, f_day * W_el,m) | 989.8 kWh | exact |
- Formula
share_h = max(0, T_base − T_h) / Σ degree hours · W_el,h = (Q_heat,h + Q_DHW,h) / SCOP · cover_h = min(PV_h, W_el,h)- Valid for
- Hourly calculation on the TMY grid (1,155 DACH grid points, PVGIS-SARAH3 2005–2023): heat demand distributed over degree hours, hot water spread evenly, electricity via a seasonal performance factor. Without a location the monthly path with an estimated daytime share remains.
- Not covered
- Monthly COP curves (in reality the COP in winter is BELOW the SCOP — so winter coverage is if anything worse), buffer store strategies, competing household consumption (here the PV is entirely available to the heat pump), utility lockout periods and a home battery.
- Data sources
- JRC Photovoltaic Geographical Information System (PVGIS), European Commission — endpoints tmy, MRcalc, printhorizon · PVGIS API v5_3, solar radiation database PVGIS-SARAH3 · retrieved 2026-07-30
Frequently asked questions
Can a PV system fully power a heat pump?
Over the year, often about half — but the annual balance is exactly the number that deceives. PV delivers in summer, the heat pump draws in winter: in the Berlin example more than 40% of the heat demand falls into the three winter months, when the PV delivers very little. That is why this calculator computes coverage per month and reports winter coverage separately.
How does the calculator know my monthly heating load?
From the location's heating degree days: for every month, the mean temperature (from the same PVGIS dataset as the irradiation) is compared against the heating base temperature — HDD = max(0, base − monthly mean) × days. The annual space-heating demand is distributed proportionally to these degree days, hot water evenly across the year. That is not a flat profile but the climatology of your location.
Why does the calculator use a single seasonal COP?
Because a monthly COP curve without flow temperature and heat source would only fake precision. What matters is the direction of the error: in reality the winter COP sits BELOW the seasonal figure — so the reported winter coverage is, if anything, still too optimistic. The SCOP itself should come from the datasheet or, better, from your own metering.
Why does coverage never rise above the daytime share?
Because without storage only the power drawn during PV hours can be covered: the calculator caps monthly coverage at the daytime share of heat-pump electricity (default 50%). That is why even July sits at 50% in the example — not for lack of PV, but because the other half of the power is needed at night. The two levers against it: a buffer tank that heats ahead at noon (raises the daytime share), or a battery.
Does a bigger PV system help winter coverage?
Less than expected: in December a system in Berlin delivers only about a tenth of its summer yield — doubling from 10 to 20 kWp doubles the December yield but still covers only part of the highest monthly demand, while the summer surplus grows into nothing. More effective for the winter balance are a better SCOP (every tenth directly cuts the electricity demand) and a higher daytime share via a buffer tank.
How does hot water enter the calculation?
Spread evenly over the twelve months — unlike the heating load, which follows the degree days. That models the summer case correctly: from June to August hot water is practically the only heat demand, and it meets the highest PV yield exactly there. Hence summer coverage reaches the full daytime-share cap in the example, while in winter heating plus hot water face the smallest yield.
How much of the heat pump electricity can my own PV really cover?
Considerably less than the annual view suggests. For 10 kWp and 14,000 kWh of heat demand near Berlin the calculator works hour by hour: 25 % of the heat pump electricity falls in hours with your own PV yield. Split by season that is 19 % in winter and 49 % in summer — and winter is exactly when the heat pump draws three quarters of its annual electricity. Of 4,000 kWh of demand, roughly 3,010 kWh still come from the grid.
Why is the coverage so much lower than often claimed?
Because the usual calculations use a blanket daytime share — often 50 %. This calculator did the same until the change, and arrived at nearly double the coverage. The daytime share assumes half the heat pump electricity falls in PV hours. In reality a heat pump draws at night and on cold, dull days — precisely when the array delivers little or nothing. Only an hourly calculation captures that.
Is a bigger PV array worth it for the heat pump?
For winter coverage, barely. In December a south-facing array in Germany delivers a fraction of its summer yield, and that is when the heat pump draws most. Doubling the array does not double winter coverage, because in many winter hours there is simply no yield to cover with. What grows is mainly the summer surplus — of little value to the heat pump, and it goes to the grid.
How does the calculator distribute heat demand across the year?
Via degree hours: for each of the 8,760 hours it takes the difference between the heating base temperature and the outdoor temperature, and distributes the annual heat demand proportionally. That is the hourly form of the familiar degree-day method. Hot water runs evenly alongside, because it does not depend on the weather. The temperature series comes from the same site dataset as the PV yield — so both curves belong to the same year.
Why does the calculator use a single seasonal performance factor?
Because a monthly COP curve would fake precision that does not exist without knowing flow temperature and heat source. What matters is the direction of the error: in reality the winter COP is BELOW the seasonal figure, because the temperature lift is larger. So winter electricity demand is if anything higher and winter coverage worse than computed here. On this point the calculator does not flatter — it is conservative.