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Air source heat pump calculator
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Air Source Heat Pump Calculator

Estimate heat pump sizing, seasonal COP, annual electricity consumption, running costs, and savings compared to your existing heating fuel.

What size air source heat pump do I need for my home?

Air source heat pump (ASHP) sizing is based on your property's design heat loss, measured in kW. This depends on floor area, insulation standard, window area, and local climate. Our calculator estimates your heat demand from floor area and insulation level, then returns the recommended heat pump capacity in kW, projected annual output, and running cost comparison versus a gas boiler - based on current UK energy tariffs and typical SCOP values.

Air Source Heat Pump Calculator

Sizing, SCOP, running cost & fuel comparison

🇬🇧 UK & Ireland
🇪🇺 Europe
🇺🇸 USA & Canada
🇦🇺 Aus & NZ
🌍 Other
★ Mild - 1,600 full-load hrs/yr
Detached
Semi-detached
Terraced
Flat
Bungalow
Total heated floor area across all storeys. For flats, use the flat's floor area only.
Poor (pre-1940s)
Average (1980s-2000)
Good (post-2000)
Excellent (EnerPHit)
■ Enter floor area to estimate heat loss
Underfloor heating (35°C)
Upgraded / large radiators (45°C)
Standard radiators (55°C)
★ Est. SCOP: 3.8

Tariffs & Costs

Unit rate from your energy bill
Full installed cost (unit + labour). Used to calculate payback.

Current Fuel Comparison

Gas
Oil
LPG
Electric
None
Mains natural gas - check your bill for exact rate

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Frequently Asked Questions

What size air source heat pump do I need?
Sizing is based on design heat loss in kW. A well-insulated modern home: 30-50 W/m² of floor area. A 100 m² well-insulated home might need a 5-7 kW heat pump. Older homes: up to 80-100 W/m² - a larger unit is needed. An accurate heat loss calculation to BS EN 12831 by an MCS-certified installer is essential before specifying.
How much does it cost to run an air source heat pump?
Running cost = heat demand ÷ SCOP x electricity tariff. A home needing 12,000 kWh/year heat with SCOP 3.0 uses 4,000 kWh of electricity - approximately £1,000/year at 25p/kWh. UK ASHPs typically achieve SCOP 2.5-3.5. Compare to gas boiler running costs of approximately £800-£1,200/year for the same heat demand at current gas prices.
Is my home suitable for an air source heat pump?
ASHPs work best in well-insulated homes (EPC C or above) with low-temperature heat emitters (underfloor heating or large radiators designed for 45-50°C flow, not the 70-80°C used with gas boilers). A fabric-first approach - improving insulation before installing the heat pump - delivers better performance and lower running costs.
Do I need planning permission for an air source heat pump?
In England, a single ASHP is permitted development (no planning permission) if: at least 1 m from the boundary; unit volume under 0.6 m³; only one unit per property; not facing a highway; not on a listed building or in a World Heritage Site. MCS installation is required for the Boiler Upgrade Scheme grant.
What is the Boiler Upgrade Scheme (BUS) grant for heat pumps?
The Boiler Upgrade Scheme (BUS) provides £7,500 off the cost of an air source heat pump in England and Wales (as of October 2023). The grant is deducted from the installer's invoice - you pay the balance. To qualify: valid EPC with no outstanding loft or cavity wall insulation recommendations; installed by an MCS-certified installer. Scheme runs until March 2028.
What is SCOP and why does it matter for heat pump running costs?
SCOP (Seasonal Coefficient of Performance) measures efficiency across the whole heating season. A SCOP of 3.0 = 3 kWh heat per 1 kWh electricity consumed. UK ASHPs typically achieve SCOP 2.5-3.5. Lowering the flow temperature (via larger radiators or underfloor heating) significantly improves SCOP. MCS sets minimum SCOP for BUS grant eligibility.

SCOP by Flow Temperature

Lower flow temperatures give much higher efficiency. SCOP (Seasonal COP) is the average over the heating season.

Flow temp Mild Average Cold
35°C (UFH)3.83.22.6
45°C (large rads)3.12.72.2
55°C (std rads)2.52.21.8

Underfloor heating or oversized / low-temperature radiators are strongly recommended for heat pump compatibility.

Heat Loss Guide

Insulation W/m²
Poor (pre-1940s)115
Solid walls, no cavity, minimal loft insulation
Average (1980s-2000)80
Cavity walls, some loft insulation
Good (post-2000)50
Well-insulated cavity, 200 mm+ loft insulation
Excellent (EnerPHit)22
Fabric-first retrofit to near-Passivhaus standard

A formal heat loss survey (to MCS or SAP) gives the most accurate figure - use the manual override if you have one.

Sizing Rule

  • ASHP output is sized to cover the design heat loss at the design outdoor temperature.
  • A 1.25x oversizing factor is applied to handle defrost cycles and brief cold snaps without using backup electric resistance heating.
  • Oversizing too much reduces part-load efficiency - avoid more than ~1.5x unless the climate is severe.
  • Common residential sizes: 5-16 kW. Larger properties or cold climates may need 20 kW+.

Distribution Systems

  • Underfloor heating (35°C): Ideal. Large surface area means very low flow temperature and maximum SCOP.
  • Upgraded / fan radiators (45°C): Standard radiators upsized by 1.5-2x or low-temperature fan convectors. Good efficiency.
  • Standard radiators (55°C): Works but significantly reduces SCOP. Best reserved where refurbishment is not possible.

Moving from 55°C to 35°C flow can improve SCOP by 35-50%, typically adding years off payback.

Key Terms

  • SCOP - Seasonal COP: average efficiency across the heating season. A SCOP of 3.5 means 3.5 kWh heat per 1 kWh electricity.
  • Design heat loss (kW) - the rate of heat loss from the property at the coldest outdoor design temperature. This drives the ASHP unit size.
  • Full-load hours - the equivalent number of hours per year the ASHP runs at full capacity. Multiplied by heat loss to give annual heat demand.
  • MCS - Microgeneration Certification Scheme. MCS-certified installers are required for UK grant eligibility (Boiler Upgrade Scheme).

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