Calculator
Heat Pump and Solar Sizing Calculator
Pairing a heat pump with solar is often presented as a natural match. Physically it is an awkward one: heating demand peaks in December and solar output bottoms out in the same month.
This calculator quantifies both sides — annual electricity demand from the heat pump, annual and winter solar supply — so the mismatch is visible rather than assumed away.
Enter the building and array
Delivered heat, not fuel input.
October to March, typically 20–30% in Europe.
Heat pump electricity for space heating
3,529 kWh/yr
Heat pump electricity for hot water
893 kWh/yr
Total heat pump electricity
4,422 kWh/yr
Annual solar generation
7,800 kWh
176% of the heat pump demand on an annual basis
Generation in the heating season
1,950 kWh
Against 3,000 kWh of winter heating electricity
Winter demand actually covered by solar
780 kWh
26% of heating-season electricity
Array to match annual demand
3.4 kWp
Annual balance only — not a winter guarantee
How the calculation works
Heat pump electricity is delivered heat divided by the seasonal coefficient of performance. Space heating and hot water are separated because hot water runs at a higher flow temperature and therefore a lower COP, often a full point below the space heating figure.
Solar supply is split the same way. Between October and March a fixed array in Europe delivers only 20 to 30% of its annual output, while 80 to 90% of the heating demand falls in that window. The mismatch is structural.
Of the winter generation that does arrive, only part can be used directly by the heat pump, because generation peaks at midday and heating demand peaks at dawn and dusk. Thermal storage in a buffer tank or in the building fabric is what raises that fraction.
The formula
E_heat = Q_space / SCOP E_dhw = Q_dhw / COP_dhw E_total= E_heat + E_dhw G = P_kWp × Y G_winter = G × s_winter Covered = min(G_winter × u_direct, E_heat × s_heating) P_needed = E_total / Y
Assumptions behind the numbers
- Heat demand is delivered heat at the emitters, which is what a heat loss calculation or an energy performance certificate provides.
- SCOP is the seasonal figure for the actual flow temperature, not the headline COP at 35°C flow and 7°C ambient.
- Seasonal split factors are typical European values; verify against monthly PVGIS output for your site.
- Direct-use share assumes some thermal buffering. Without any storage it is closer to 25%; with a well-controlled buffer tank and weather compensation, 50% is achievable.
- No battery is modelled. A battery shifts a few hours, not a season, so it changes this result far less than owners expect.
How to read the result
An array that matches heat pump demand on an annual basis still imports most of the winter electricity and exports most of the summer surplus. That is a valid arrangement where net metering or a fair export price exists, and a poor one where export is unpaid.
The number to watch is winter coverage. Twenty to forty percent of heating-season electricity from solar is a realistic ceiling for a fixed array without seasonal storage.
Steeper array tilts help. Raising tilt from 30° to 55° costs a few percent of annual yield but can add 15 to 25% to December output, which is exactly when the heat pump needs it.
Worked example
A house needs 12,000 kWh of space heat and 2,500 kWh of hot water, with a SCOP of 3.4 and a hot water COP of 2.8, alongside a 6 kWp array yielding 1,300 kWh/kWp.
Electricity demand is 3,529 kWh for heating and 893 kWh for hot water, or 4,422 kWh in total. The array generates 7,800 kWh a year — 176% of the heat pump demand on paper.
In the heating season the array produces 1,950 kWh against 3,000 kWh of heating electricity, and only about 40% of that generation lands when the heat pump can use it: 780 kWh, or 26% of winter heating electricity. The annual surplus is real, but it arrives in July.
Mistakes that skew the answer
- Comparing annual generation with annual heat pump demand and concluding the heating is solar-powered.
- Using the headline COP instead of SCOP at the design flow temperature, which understates electricity demand by 20 to 30%.
- Ignoring hot water, which runs all year and is the one heat pump load solar covers well in summer.
- Sizing the array to winter demand, which produces an enormous summer surplus that is often exported for very little.
- Assuming a home battery solves the seasonal gap. It shifts hours, not months.
Questions readers ask
- How many solar panels does a heat pump need?
- To match annual electricity on paper, roughly 3 to 5 kWp for a typical European house needing 4,000 to 5,000 kWh of heat pump electricity. To cover a meaningful share of winter demand you would need several times that, which is why annual matching is the practical target.
- Does solar make a heat pump cheaper to run?
- Yes, but mostly through summer hot water and shoulder-season heating, plus the export income that offsets winter imports. Expect a 20 to 35% reduction in heat pump running costs from a well-sized array, not elimination.
- Should I tilt the array more steeply for winter?
- If heating dominates the load, yes. A tilt around latitude plus 15° maximises winter output at a small annual cost. Check row spacing and wind loading, both of which worsen with steeper tilts.
- Is a buffer tank worth it?
- For solar matching, usually. A 200 to 300 litre buffer lets the heat pump run at midday when solar is available and release heat into the evening, raising direct use from roughly 25% to 45%. It also reduces compressor cycling, which helps the SCOP.
- What about a hot water diverter?
- Diverters send surplus to an immersion heater at a COP of 1, which is thermodynamically wasteful when a heat pump at COP 2.8 is available. Prefer controls that raise the heat pump's hot water setpoint during surplus periods.