Introduction
This case study compares three example properties of different ages and insulation standards. Each property is equipped with a south facing solar PV array, a battery and utilise an economy 7 tariff. What differs between each property is the insulation standard which changes the heat loss and the assumed radiator flow temperature, with high flow temperatures assumed for older properties.
The pre-1920s home has much higher heat loss, so it uses far more electricity for heating. Even so, the solar and battery system still creates a large modelled saving by benefiting from cheap overnight electricity in the winter and solar energy throughout the rest of the year. The newer and low-carbon homes are cheaper to run because they need less heat and can operate at lower flow temperatures.
These reports were modelled using our solar calculator: open the free Solar Butter solar calculator, which is free to use with no sign up required.
Assumptions
- UK weather data for 2025, using Gloucester as an example location
- 17 south-facing Aiko 485 W panels, about 8.2 kW PV in total
- 5 kW inverter and 32 kWh battery
- Economy 7 tariff with 13p and 30p import rates, 12p export, and 60p/day standing charge
- 6,000 kWh/year household load before heat pump electricity is added
- Weather compensation enabled in each heat pump model
- Hot water usage estimated using an occupancy of 3
- Property floor area of 120 m2
- SCOP figures taken from the Vaillant aroTHERM plus installer quick guide
The reports are modelled estimates only. Real home heat loss needs to be estimated from a heat loss survey or through consumption data. Tariff is subject to availability.
Scenario 1: Pre-1920s house
Inputs
- Pre-1920s property with 9.45 kW design heat loss at -3 C
- 55 C design flow temperature and 40 C minimum flow temperature
Outputs
- 6,816 kWh/year space heating electricity
- 976 kWh/year hot-water electricity
- 7,792 kWh/year total heat pump electricity
- 29,928 kWh/year total thermal output
- Average combined COP of 3.84
- £990/year modelled bill with PV, battery, export credit, and standing charge
- £2,714/year total benefit from solar and battery
The modelled heat pump still performs with a combined COP above 3.5, but the property asks for a lot of heat. The high total thermal demand dominates the bill, even though the time-of-use tariff and large battery reduce the cost of imported electricity.
Scenario 2: Post-2010 house
Inputs
- Post-2010 property with 2.89 kW design heat loss at -3 C
- 45 Deg C design flow temperature and 30 C minimum flow temperature
Outputs
- 1,335 kWh/year space heating electricity
- 1,032 kWh/year hot-water electricity
- 2,367 kWh/year total heat pump electricity
- 8,257 kWh/year total thermal output
- Average combined COP of 3.49
- £169/year modelled bill with PV, battery, export credit, and standing charge
- £2,316/year total benefit from solar and battery
The newer property is dramatically cheaper to run because the building needs much less heat. Its combined COP is lower than the pre-1920s case in this updated model because hot water is a larger share of the annual heat demand, but the annual heat demand is far lower. That is why insulation has such a large effect on heat pump running costs.
Scenario 3: Low-carbon underfloor house
Inputs
- New low-carbon property with 2.25 kW design heat loss at -3 C
- 30 C design flow temperature and 20 C minimum flow temperature
- Underfloor-style low temperature heating, three occupants, and a 200 litre hot-water tank
Outputs
- 625 kWh/year space heating electricity
- 1,060 kWh/year hot-water electricity
- 1,685 kWh/year total heat pump electricity
- 6,358 kWh/year total thermal output
- Average combined COP of 3.77
- £84/year modelled bill with PV, battery, export credit, and standing charge
- £2,260/year total benefit from solar and battery
This is the best case in the comparison. Low heat loss reduces the energy needed, and the 30 C design flow temperature lets the heat pump work more efficiently. The annual bill is almost cancelled out by export income after the battery has shifted cheap overnight energy and solar generation through the day.
Comparison
£990/year bill
High heat loss and 55 C flow temperature make this the most expensive case.
£169/year bill
Lower heat loss cuts heat pump electricity to 2,367 kWh/year.
£84/year bill
Underfloor heating and low heat loss give the lowest bill.
The old house is not automatically a bad fit for a heat pump. The modelled solar and battery benefit is actually largest in that scenario because the home has more electricity demand to offset. The problem is that a high heat-loss property still has a much larger bill after those savings have been applied.
For larger properties the COP can increase slightly because hot water is a smaller share of the total thermal output, and hot water is usually produced at a higher temperature than space heating. That does not cancel out the cost of poor fabric performance: total heat demand still matters most.
| Scenario | Design heat loss | Design flow | Heat pump electricity | Thermal output | Space heating COP | Hot water COP | Combined COP | Grid import / export | New bill | Solar + battery benefit |
|---|---|---|---|---|---|---|---|---|---|---|
| Pre-1920s house | 9.45 kW | 55 C | 7,792 kWh/year | 29,928 kWh/year | 3.95 | 3.08 | 3.84 | 7,041 / 2,075 kWh | £990/year | £2,714/year |
| Post-2010 house | 2.89 kW | 45 C | 2,367 kWh/year | 8,257 kWh/year | 3.94 | 2.90 | 3.49 | 3,151 / 3,699 kWh | £169/year | £2,316/year |
| Low-carbon underfloor house | 2.25 kW | 30 C | 1,685 kWh/year | 6,358 kWh/year | 5.42 | 2.80 | 3.77 | 2,667 / 3,904 kWh | £84/year | £2,260/year |
SCOP and Unit Size Considerations
For older properties with higher heat demands, this often corresponds to higher radiator flow temperatures and higher heat pump power requirements. Higher flow temperatures are associated with lower efficiency due to thermodynamics. However, the larger heat pump models are correlated with higher efficiency and SCOP in general. Slightly offsetting the drop in thermodynamic performance. This can be seen from the SCOP figures from Vaillant Group UK's Installer's Quick Guide - aroTHERM plus 03/2024.
| Heat pump size | SCOP at 35 C flow | SCOP at 40 C flow | SCOP at 45 C flow | SCOP at 50 C flow | SCOP at 55 C flow |
|---|---|---|---|---|---|
| 3.5 kW | 4.41 | 4.03 | 3.65 | 3.37 | 3.10 |
| 5 kW | 4.48 | 4.13 | 3.77 | 3.41 | 3.06 |
| 7 kW | 4.36 | 4.13 | 3.91 | 3.65 | 3.39 |
| 10 kW | 5.03 | 4.58 | 4.13 | 3.85 | 3.58 |
| 12 kW | 4.88 | 4.55 | 4.21 | 3.92 | 3.63 |
This manufacturer table gives two insights. Firstly, higher flow temperatures are associated with a drop in SCOP due to thermodynamics. Second, higher power units are associated with slightly increased SCOP generally.
Solar and batteries help all three homes. The older property still presents considerable savings over a grid-only setup when a time-of-use tariff such as Economy 7 or an EV tariff is used, because the battery can buy cheaper overnight electricity and reduce peak imports.
Conclusion
Heat pumps can be beneficial for old properties, especially when they are paired with solar panels, battery storage, and a time-of-use tariff. In this model, the pre-1920s home still gets a £2,714/year solar and battery benefit, and the heat pump delivers nearly four units of heat for each unit of electricity on average.
But the newer homes are clearly better from a running-cost point of view. High levels of insulation reduce the amount of heat needed, and lower radiator or underfloor flow temperatures improve heat pump efficiency. The low-carbon underfloor case lands at only £84/year in this model, compared with £990/year for the pre-1920s house.
For context, the pre-1920s property needs 29,928 kWh/year of heat in this model. With a gas boiler at 90% efficiency, that would require about 33,253 kWh/year of gas. At £0.06/kWh, the gas energy cost would be about £1,995/year. The same property also has 6,000 kWh/year of non-heating electricity load; at £0.30/kWh that adds £1,800/year, giving about £3,795/year before standing charges. On that basis, the modelled heat pump, solar, battery, and Economy 7 setup is roughly £2,805/year cheaper than gas plus standard-rate household electricity.
The practical conclusion is that an old-house heat pump can still make strong financial sense when it is designed as part of a whole-home energy system. In this model, the older property is still far cheaper than gas heating plus standard-rate household electricity once solar, battery storage, and a time-of-use tariff are included. The best results still come from improving fabric where sensible, checking radiator sizing, using weather compensation, and lowering flow temperatures where the emitters allow.
