Heat Pump Calculator
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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.
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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