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How to Size a Heat Pump
Correct sizing is critical for heat pump performance. An undersized heat pump cannot heat your home on cold days. An oversized heat pump short-cycles, reducing efficiency and lifespan. This guide explains the sizing formula and provides reference tables for common home types.
The Heat Pump Sizing Formula
Heat pump size (kW) = (Home area x Heat loss factor) / 1000 x 1.2 safety margin
The heat loss factor depends on your home's insulation, age, and construction type. The 1.2 safety margin ensures the heat pump can meet demand on the coldest days without relying on backup electric heating.
Sizing by Home Type
| Home Type | Area (sqm) | Insulation | Heat Loss (W/sqm) | Heat Pump Size (kW) |
|---|---|---|---|---|
| Modern flat (2020+) | 65 | Excellent | 40 | 3.1 |
| 1990s semi-detached | 85 | Good | 70 | 7.1 |
| 1970s detached | 120 | Average | 100 | 14.4 |
| Victorian terraced | 90 | Below average | 140 | 15.1 |
| Period farmhouse | 200 | Poor | 180 | 43.2 |
Homes with poor insulation need very large (and expensive) heat pumps. Improving insulation before installing a heat pump is almost always the better investment. Reducing heat loss from 140 W/sqm to 80 W/sqm can halve the required heat pump size and save thousands on the installation.
Running Cost Comparison
| Fuel Type | Cost/kWh | System Efficiency | Effective Cost/kWh Heat |
|---|---|---|---|
| Gas boiler (condensing) | EUR 0.065 | 90% | EUR 0.072 |
| Gas boiler (old) | EUR 0.065 | 75% | EUR 0.087 |
| Oil boiler | EUR 0.085 | 85% | EUR 0.100 |
| Electric heaters | EUR 0.245 | 100% | EUR 0.245 |
| LPG boiler | EUR 0.120 | 85% | EUR 0.141 |
| Heat pump (SCOP 2.8) | EUR 0.245 | 280% | EUR 0.088 |
| Heat pump (SCOP 3.5) | EUR 0.245 | 350% | EUR 0.070 |
A heat pump with a SCOP of 2.8 (a conservative estimate for air source) has a running cost similar to a modern gas boiler. With underfloor heating (which allows higher flow temperatures), SCOP can reach 3.5+, making the heat pump significantly cheaper to run. Pairing a heat pump with solar panels can further reduce running costs by generating free electricity during the day.
Annual Heating Cost by Home Size
| Home Size | Heating Demand | Gas Boiler | Oil Boiler | Heat Pump (2.8) | Heat Pump + Solar |
|---|---|---|---|---|---|
| 60 sqm | 8,400 kWh | EUR 605 | EUR 840 | EUR 735 | EUR 515 |
| 100 sqm | 14,000 kWh | EUR 1,010 | EUR 1,400 | EUR 1,225 | EUR 858 |
| 150 sqm | 21,000 kWh | EUR 1,515 | EUR 2,100 | EUR 1,838 | EUR 1,287 |
| 200 sqm | 28,000 kWh | EUR 2,020 | EUR 2,800 | EUR 2,450 | EUR 1,715 |
Based on 70 W/sqm heat loss, 2000 heating hours/year. Heat pump + solar assumes 30% of electricity from solar panels.
The biggest savings come from replacing oil or LPG heating. Replacing a modern gas boiler with a heat pump saves less on running costs but significantly reduces CO2 emissions and future-proofs against gas price rises.
How Much Does a Heat Pump Cost?
Heat pump costs vary significantly based on the system type, home size, and installation complexity. This section breaks down every cost component, compares air source and ground source options, and shows the 10-year total cost of ownership compared to conventional heating.
Installation Cost by System Type
| System Type | Unit Cost | Installation | Total Installed | Best For |
|---|---|---|---|---|
| Air source (5-8 kW) | EUR 3,500-5,500 | EUR 3,500-5,500 | EUR 7,000-11,000 | Flats, small homes |
| Air source (8-14 kW) | EUR 5,000-8,000 | EUR 4,000-7,000 | EUR 9,000-15,000 | Average homes |
| Air source (14-20 kW) | EUR 7,000-11,000 | EUR 5,000-8,000 | EUR 12,000-19,000 | Large or poorly insulated |
| Ground source (borehole) | EUR 8,000-12,000 | EUR 10,000-18,000 | EUR 18,000-30,000 | Rural with garden space |
| Ground source (trench) | EUR 8,000-12,000 | EUR 7,000-12,000 | EUR 15,000-24,000 | Large gardens |
Air source heat pumps are the most popular choice for retrofits because they are less expensive and do not require garden excavation. Ground source heat pumps offer higher efficiency (SCOP 3.5-4.5 vs 2.5-3.5) but the extra cost rarely pays back in running cost savings alone.
Cost Breakdown
| Component | Share of Total | Typical Range |
|---|---|---|
| Heat pump unit | 50-60% | EUR 3,500-11,000 |
| Installation labour | 20-25% | EUR 2,000-5,000 |
| Pipework and fittings | 10-15% | EUR 800-2,500 |
| Controls and thermostat | 3-5% | EUR 300-800 |
| Commissioning and certification | 3-5% | EUR 300-600 |
| Electrical supply upgrade | 0-5% | EUR 0-500 |
Grants and Subsidies
| Country | Scheme | Grant Amount | Net Cost (8kW ASHP) |
|---|---|---|---|
| UK | Boiler Upgrade Scheme | GBP 7,500 | GBP 3,000-6,500 |
| Ireland | SEAI Heat Pump Grant | EUR 3,500-6,500 | EUR 4,500-8,500 |
| Germany | BEG Funding | Up to 40% of cost | EUR 6,000-9,000 |
| France | MaPrimeRenov | EUR 3,000-5,000 | EUR 6,000-10,000 |
| Italy | Superbonus / Ecobonus | 50-65% tax deduction | EUR 3,500-7,500* |
| Netherlands | ISDE subsidy | EUR 3,000-4,500 | EUR 6,500-10,500 |
*Italian deductions are spread over 10 years of tax returns, not upfront savings.
10-Year Total Cost Comparison
| Heating System | Install Cost | Annual Running | Annual Service | 10-Year Total |
|---|---|---|---|---|
| Gas boiler (new) | EUR 3,000 | EUR 1,010 | EUR 80 | EUR 13,900 |
| Oil boiler (new) | EUR 4,000 | EUR 1,400 | EUR 120 | EUR 19,200 |
| ASHP (no grant) | EUR 11,000 | EUR 1,225 | EUR 150 | EUR 24,750 |
| ASHP (with EUR 5,000 grant) | EUR 6,000 | EUR 1,225 | EUR 150 | EUR 19,750 |
| ASHP + solar panels | EUR 17,000 | EUR 858 | EUR 200 | EUR 27,580 |
Based on 100 sqm home, 14,000 kWh annual heating demand, 70 W/sqm heat loss.
After grants, an air source heat pump reaches cost parity with gas heating within 10 years for most homes. The heat pump then continues operating for another 10-15 years with minimal maintenance. Enter your home's details in the calculator above for a personalised cost estimate, and check if improving your insulation first can reduce the required system size.
These calculations are estimates only. Actual requirements may vary depending on surface conditions, product specifications, and installation methods. Always consult a qualified professional for precise measurements.
Prices updated: 2026-03Frequently Asked Questions
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