Heat pumps have become the default heating replacement technology in Europe, Australia, and increasingly in North America. When paired correctly with a solar system, they become a lever for dramatically reducing both carbon emissions and energy bills. When sized incorrectly, the mismatch between peak heat demand and peak solar generation forces expensive grid import.
The Physics: Why Solar and Heat Pumps Work Together
A heat pump moves thermal energy rather than generating it, achieving a coefficient of performance (COP) of 2.5–5.0 depending on outdoor temperature and target flow temperature. This means:
- 1 kWh electricity in → 3–4 kWh heat delivered (typical COP of 3–4)
- A 6 kWh solar surplus on a spring afternoon → 18–24 kWh of heating if directed to the heat pump
Modern air-source heat pumps (ASHPs) draw between 0.8 kW and 3.5 kW depending on their rated output and operating conditions. This is well within the power range of a residential solar system.
Step 1: Calculate Annual Heat Demand
Your heat demand depends on your home's heat loss rate (measured in watts per degree Kelvin, W/K or kW/K) and your local heating degree days (HDD).
Simplified formula: Annual heat demand (kWh) = Heat loss rate (W/K) × Annual HDD × 24 ÷ 1000
Example: A moderately insulated semi-detached home with a heat loss of 80 W/K in a region with 2,500 HDD: 80 × 2,500 × 24 ÷ 1,000 = 4,800 kWh/year heating demand
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For homes with good insulation (EPC B or above), typical annual heat demand is 3,000–6,000 kWh. Older, poorly insulated homes may demand 12,000–20,000 kWh.
Step 2: Calculate Heat Pump Electricity Consumption
Divide annual heat demand by the seasonal COP (SCOP) — the average COP across the heating season, which accounts for colder days when COP is lower:
Heat pump electricity (kWh/yr) = Heat demand ÷ SCOP
For a 4,800 kWh demand home with SCOP of 3.2: 4,800 ÷ 3.2 = 1,500 kWh/year electricity for heating
This is a manageable addition to a residential electricity budget and represents an excellent solar self-consumption opportunity.
Step 3: Size the Solar System for the Combined Load
Add your heat pump electricity consumption to your existing household consumption:
- Existing consumption: 4,500 kWh/year
- Heat pump addition: 1,500 kWh/year
- Total: 6,000 kWh/year
Using local peak sun hours (available from PVGIS for any location), calculate required system size: System kWp = Annual kWh ÷ (Peak sun hours × 365 × efficiency factor)
For a 1,200 peak sun hours location with 80% system efficiency: 6,000 ÷ (1,200 × 365 × 0.80) ÷ 1,000 = 6,000 ÷ 350,400 → approximately 5.7 kWp
Round to the next standard system size: 6 kWp is appropriate.
Step 4: Time-Shifting Heat Demand
The real optimisation opportunity is shifting heat pump operation into peak solar generation hours (typically 09:00–15:00). Modern smart heat pumps from Vaillant, Daikin, Samsung, and Mitsubishi Electric offer:
- Scheduled operation: Force the heat pump to run during solar generation hours
- Legionella cycle scheduling: Move the hot water pasteurisation cycle to midday
- Weather compensation: Reduce flow temperature on mild sunny days, lowering electricity draw
- Smart home integration: Integrate with inverter output via Home Assistant, SMA Energy Meter, or Sungrow export signal
A GivEnergy or Sungrow inverter with a CT clamp can send a signal to a compatible heat pump controller when solar surplus exceeds a threshold — triggering the heat pump to run on clean solar power rather than grid electricity.
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Comparison: Self-Consumption Rate With and Without Heat Pump Scheduling
| Scenario | Solar Generated (kWh/yr) | Self-Consumed | Export |
|---|---|---|---|
| 6 kWp, no heat pump | 7,200 | 3,800 (53%) | 3,400 (47%) |
| 6 kWp + heat pump (default schedule) | 7,200 | 5,100 (71%) | 2,100 (29%) |
| 6 kWp + heat pump (solar-scheduled) | 7,200 | 6,100 (85%) | 1,100 (15%) |
Scheduling the heat pump to run during solar generation hours increases self-consumption by 14 percentage points in this example — directly improving financial returns without any additional hardware.
Battery Storage Consideration
If you are already planning a battery, a heat pump reduces the case for large storage. With a scheduled heat pump absorbing midday surplus, you need less battery capacity to shift remaining surplus to the evening. In many cases, a 6 kWp solar system paired with a well-scheduled heat pump and a 5 kWh battery outperforms the same system with a 10 kWh battery and an unscheduled heat pump.