A heat pump burns nothing. It uses electricity to run a compressor that moves heat already present in outdoor air, the ground or water into your home. Moving heat takes far less energy than making it, so a heat pump delivers three to four units of heat for every unit of electricity. That is why every serious plan to decarbonise buildings relies on heat pumps, and it is also why badly designed installations disappoint. This guide covers the physics, the efficiency figures that actually predict your bill, how to size a system, and how to compare running costs fairly against gas, oil or electric resistance heating.
How a heat pump works
A fridge and a 20 MW district heating unit run the same vapour-compression cycle:
- Evaporator. Refrigerant colder than the outdoor air (or ground) flows through a heat exchanger. It absorbs heat and evaporates even when the air is at −10 °C, because refrigerants such as R290 (propane) or R32 boil at very low temperatures.
- Compressor. The electric compressor squeezes the vapour, raising its temperature to 50–80 °C.
- Condenser. The hot vapour gives up its heat to your heating water (air-to-water) or indoor air (air-to-air) and condenses back into liquid.
- Expansion valve. Pressure drops, the refrigerant cools sharply, and the cycle starts again.
Electricity runs the compressor and fans. The rest of the delivered heat comes free from the environment, which is why EU law counts heat pumps as renewable heat, provided their seasonal efficiency is high enough.
Main types
| Type | Heat source | Typical SCOP (central Europe) | Installed cost, single-family home | Notes |
|---|---|---|---|---|
| Air-to-water (monobloc or split) | Outdoor air | 2.8–4.0 | €9,000–18,000 | Most common in Europe; feeds radiators or underfloor heating and hot water |
| Air-to-air (split / multi-split) | Outdoor air | 3.5–5.0 (heating) | €2,000–8,000 | Cheapest; also cools; no hot water |
| Ground-source (borehole or horizontal loop) | Ground at 8–12 °C | 3.5–4.8 | €18,000–35,000 | Stable source temperature; drilling is the main cost |
| Water-source | Lake, river, groundwater | 4.0–5.0 | Site-specific | Needs permits and a suitable source |
| Exhaust-air | Ventilation exhaust | 2.5–3.5 | €7,000–12,000 | Suits small, airtight homes |
Costs are indicative for European installations in 2024–2026, before subsidies. The UK, Germany and Scandinavia sit at the upper end, Eastern Europe lower. To size borehole length for your soil, use the ground source heat pump calculator.
COP, SCOP and SPF: the numbers that matter
COP (coefficient of performance) is the instantaneous ratio of heat delivered to electricity used:
Carnot limit: COP_max = T_hot / (T_hot − T_cold) (temperatures in kelvin)
The Carnot formula shows that temperature lift matters more than anything else. Heating water to 35 °C (308 K) from 7 °C air (280 K) gives a theoretical maximum of 308 / 28 ≈ 11. Real machines reach roughly 40–55% of Carnot, so the COP lands around 4.5–5. Raise the flow temperature to 55 °C and the theoretical limit drops to about 6.8, with real COP around 3. Each 1 °C off the flow temperature improves efficiency by roughly 2–3%.
SCOP (seasonal COP) is a standardised EN 14825 figure. It weights COP across a modelled heating season for a reference climate (average, colder or warmer) at a set flow temperature: 35 °C “low temperature” or 55 °C “medium temperature”. You’ll find it on the EU energy label. It is good for comparing products, but it remains a lab estimate.
SPF (seasonal performance factor) is what you measure in a real home over a year: total heat out divided by total electricity in, with defrost cycles, circulation pumps, backup heaters and domestic hot water all included. Field studies are the fair benchmark. Fraunhofer ISE monitored heat pumps for several years in existing, partly renovated German homes. Air-source SPFs fell mostly between about 2.6 and 4.0, averaging around 3.1–3.4, and ground-source systems averaged around 4. Systems with low flow temperatures were always near the top.
| Flow temperature at design conditions | Typical air-source SPF | Typical ground-source SPF |
|---|---|---|
| 35 °C (underfloor heating) | 3.5–4.2 | 4.3–5.0 |
| 45 °C (large radiators) | 3.0–3.6 | 3.8–4.5 |
| 55 °C (standard radiators) | 2.5–3.1 | 3.3–3.9 |
| 65 °C+ (unmodified old system) | 2.0–2.6 | 2.8–3.3 |
Domestic hot water is heated to 50–55 °C all year, usually at a COP of 2.5–3.2. In well-insulated homes, where hot water is a large share of demand, it pulls the annual figure down a little.
Running costs: the price-ratio rule
Three things set your heating bill: how much heat the house needs, how efficient the heating system is, and what the energy costs. To compare fairly with a gas boiler:
Annual cost (gas boiler) = Heat demand (kWh) / η_boiler × gas price + standing charge
Break-even price ratio = SPF / η_boiler
Take an SPF of 3.2 and a boiler efficiency of 0.90. That is realistic for a condensing boiler in an existing home, unlike the 98% printed on the box. The heat pump then wins whenever electricity costs less than 3.2 / 0.9 ≈ 3.6 times the gas price per kWh. If you can drop the gas connection altogether, the standing charge you save (often €100–300 a year) strengthens the case.
| Country (household prices, approx. 2025) | Electricity/gas price ratio | Heat pump vs. gas at SPF 3.2 |
|---|---|---|
| UK | ~4.0 | Roughly break-even; wins with a heat pump tariff |
| Germany | ~3.0–3.5 | Heat pump cheaper |
| France | ~2.0–2.3 | Heat pump clearly cheaper |
| Sweden, Norway | Gas largely unavailable | Heat pumps dominate |
| Romania, Hungary | ~4–6 (regulated gas) | Gas often cheaper on running cost |
Ratios shift with tariffs and policy. Check current prices for your country on our country pages and run the numbers in the heat pump vs gas boiler calculator.
Worked example: 140 m² house, Germany
- Annual space heating + hot water demand: 15,000 kWh of heat (a partly renovated 1980s house).
- Air-to-water heat pump, radiators at 50 °C design flow, measured SPF 3.3.
- Electricity €0.34/kWh (heat pump tariff ~€0.28/kWh); gas €0.11/kWh plus €150/year standing charge; boiler seasonal efficiency 0.90.
Gas boiler: 15,000 / 0.90 = 16,667 kWh gas × €0.11 = €1,833 + €150 = €1,983/year.
Heat pump (standard tariff): 15,000 / 3.3 = 4,545 kWh × €0.34 = €1,545/year, saving €438.
Heat pump (heat pump tariff): 4,545 × €0.28 = €1,273/year, saving €710.
Say the heat pump costs €12,000 net after a 30–50% subsidy and a replacement gas boiler €4,000. The extra €8,000 pays back in roughly 11–18 years on energy alone. Rising carbon prices shorten that (the EU ETS2 covers building fuels from 2027–2028). Rooftop solar moves some consumption onto free daytime power; the solar panel calculator shows how much. Plug in your own figures in the heat pump calculator.
Sizing: the most common mistake
An installer who swaps a 24 kW boiler for a 16 kW heat pump “to be safe” creates expensive problems. Oversized units cycle on and off, run at poorer part-load efficiency and cost more up front. Boilers were routinely oversized two to three times because it cost almost nothing. A heat pump should match the building’s design heat loss:
U = thermal transmittance (W/m²K), A = element area (m²) n = air changes per hour, V = heated volume (m³)
Rules of thumb for specific heat load at a −10 °C design temperature:
| Building standard | Specific heat load | 140 m² house |
|---|---|---|
| Passive house | 10–15 W/m² | 1.5–2 kW |
| New build (post-2015 EU) | 25–40 W/m² | 3.5–5.5 kW |
| Renovated 1970s–1990s | 50–70 W/m² | 7–10 kW |
| Unrenovated pre-1970 | 90–130 W/m² | 13–18 kW |
For an existing home there is a quick shortcut. Take the last three years of gas use, multiply by boiler efficiency to get heat demand, then divide by the local full-load heating hours (about 1,800–2,400 h in central Europe). 20,000 kWh gas × 0.9 / 2,000 h ≈ 9 kW.
Cut demand before you size. Loft and cavity wall insulation usually reduce heat loss by 20–35%, which lets you buy a smaller, cheaper unit that runs at a lower flow temperature. See our home insulation guide and the insulation savings calculator.
Getting high efficiency in practice
- Lower the flow temperature to the minimum that keeps rooms comfortable, and turn on weather compensation so the flow temperature follows the outdoor temperature.
- Run it low and slow. Heat pumps are most efficient running continuously at low output. Deep night setbacks force a high-temperature recovery in the morning.
- Open the system up. Take out thermostatic valves you don’t need in the main living areas, and skip buffer tanks unless the hydraulics demand one. Every extra heat exchange or mixing step costs efficiency.
- Upgrade the weakest radiators. Two or three rooms often set the required flow temperature. Larger or fan-assisted radiators in those rooms can bring the system temperature down by 5–10 °C.
- Watch the backup heater. An electric immersion or flow heater running at COP 1 can quietly wreck seasonal performance. Let it switch on only below the bivalent point.
- Monitor. A heat meter and an electricity sub-meter cost a few hundred euros and show your real SPF.
Climate impact
A heat pump’s emissions depend on its electricity. At the 2025 world average grid intensity of roughly 460 g CO₂/kWh (Ember / Our World in Data), an SPF of 3.2 gives about 145 g CO₂ per kWh of heat. A gas boiler emits about 225 g (≈ 202 g/kWh of gas at 90% efficiency). Cleaner grids widen the gap dramatically: in France (~40 g/kWh) the heat pump drops to about 13 g/kWh of heat. A low-efficiency heat pump only comes close to gas on very coal-heavy grids, above roughly 650–700 g/kWh. Check your country on the carbon intensity of electricity page and with the electricity CO₂ calculator.
The IEA estimates that heat pumps deployed in line with national pledges could cut global CO₂ emissions by at least 500 million tonnes by 2030. By its figures they emit at least 20% less than gas boilers even on gas-heavy grids, and up to 80% less on clean grids.
Common myths
- “Heat pumps don’t work in the cold.” Norway, Sweden and Finland have Europe’s highest heat pump penetration. Efficiency drops in cold weather, but the units keep running; see heat pumps in cold climates.
- “You need underfloor heating.” It helps but isn’t required. Most radiator systems work at 45–55 °C after modest upgrades.
- “You need a passive house.” A correctly sized heat pump can heat any house. Insulation just makes it cheaper to buy and run.
- “They’re just electric heaters.” A resistance heater has a COP of 1. A heat pump delivers 2.5–5 times as much heat per kWh.
Bottom line
No electric heating is more efficient than a heat pump, and across most of Europe and North America a good installation already costs less to run than gas. The economics depend on two numbers you can influence, the flow temperature (which sets your SPF) and your electricity tariff, and one you mostly can’t: the gas-to-electricity price ratio. Get a proper heat-loss calculation, fix the obvious insulation gaps first, and compare quotes on guaranteed seasonal performance rather than peak COP. Then weigh it against every alternative in the heating cost comparison.
Calculators for this topic
Frequently asked questions
What is a good COP for a heat pump?
Is a heat pump cheaper to run than a gas boiler?
Do heat pumps work with radiators?
How long do heat pumps last?
What size heat pump do I need?
Are heat pumps noisy?
Sources & references
- IEA (2022). The Future of Heat Pumps
- Fraunhofer ISE — Field measurements of heat pumps in existing buildings (WPsmart im Bestand)
- EN 14825 — Air conditioners, liquid chilling packages and heat pumps: testing and rating at part load conditions and calculation of seasonal performance
- Energy Saving Trust — Electrification of Heat UK demonstration project
- IPCC AR6 Working Group III, Chapter 9 — Buildings
Figures in this guide are typical ranges drawn from the sources above; your own numbers depend on location, prices and equipment. Use the linked calculators for site-specific estimates.