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Heat Loss Calculator UK — Heat Pump Sizing
Estimate your home's heat loss in kilowatts — the single most important number for sizing an air source heat pump. UK fabric U-values, floor area, glazing and infiltration. Use before getting quotes.
Interactive heat loss calculator
Worked example
Two-storey 1930s semi, 95 m², uninsulated cavity walls, 100 mm loft, double glazing (pre-2002), Midlands:
- Whole-house heat loss
- 10.7 kW
- Recommended heat pump
- 12.0 kW
- Design ΔT
- 24°C
- BUS grant eligible
- Yes
- Walls contribute
- 35%
- Roof contributes
- 5%
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Under 8 kW? You're in Boiler Upgrade Scheme territory
The £7,500 grant usually covers 40–60% of an ASHP install for homes in this heat-loss band. Check eligibility in 60 seconds.
Check BUS grant →How heat loss is calculated
Heat loss is the rate at which your home leaks warmth to the outside, measured in watts or kilowatts. The formula for each element of the fabric is Q = U × A × ΔT — where U is the thermal conductivity in W/m²K, A is the surface area in m², and ΔT is the difference between your indoor target (21°C) and the outdoor design temperature (−2°C to −5°C depending on UK region). Add walls, roof, floor, glazing, plus a ventilation term that depends on air changes per hour × building volume × 0.33 — and you get whole-house heat loss.
That single number — usually 4 to 12 kW for a UK home — sets the entire design of a heat pump system. The unit you pick, the radiators you keep or upgrade, the flow temperature you need, the grant you qualify for, the running cost: all of it flows from heat loss. Which is why getting it roughly right before booking installer visits saves you from being oversold a 14 kW monobloc because your house "feels big."
What v1 doesn't model
- Room-by-room. MCS-accredited designers must do each room separately to size individual radiators. This calculator is whole-house only.
- Thermal bridging. Lintels, junctions and balconies add ~15% to fabric loss in older homes; we don't capture this. Consider it a small margin the recommendation already absorbs.
- Solar gain. South-facing glazing in winter slightly offsets heat loss. We ignore it — it's usually under 5% of the total.
- Hot water load. A heat pump also has to heat domestic hot water (typically adding 1–1.5 kW to peak demand). This affects cylinder sizing, not unit sizing.
- Custom U-values. We use standard SAP-assumed values; an MCS heat loss survey will measure or calculate yours.
Frequently asked questions
- How accurate is a whole-house heat loss calculator vs a room-by-room MCS survey?
- Expect ±15–20% on a well-insulated home, ±25% on a solid-wall home. MCS-compliant surveys do fabric and ventilation for every room and account for airtightness tests, which this doesn't. Use this to go into installer quotes informed — not to commit to a system size. Once you have a kW figure, run the [heat pump vs gas boiler calculator to see the annual running cost.
- What are U-values and why do they matter?
- A U-value measures how quickly heat escapes through a wall, roof or window, in watts per square metre per degree. Lower is better. A solid Victorian brick wall is ~2.1 W/m²K; a modern cavity wall ~0.3; a fully insulated new-build ~0.18. Multiply U × surface area × design temperature difference (20°C inside, -3°C outside for most of the UK) to get heat loss in watts.
- Do I need a heat pump as big as my current gas boiler?
- Almost never. A typical UK home has a 24–30 kW gas combi, but heat loss is usually 5–10 kW. Gas boilers are oversized because they need to heat hot water on demand. A heat pump is sized for the heat loss and uses a hot water cylinder — so a 7 kW heat pump replaces a 30 kW boiler comfortably. Check if you qualify for £7,500 off with the [Boiler Upgrade Scheme grant calculator.
- Why does UK design temperature matter?
- It's the outdoor temperature your system must keep up at. Most of the UK uses -3°C (England lowlands) to -5°C (Scotland, uplands). Some MCS designers use a slightly tougher -2°C to allow margin. A lower design temp means a bigger calculated heat loss.
- How does insulation change the answer?
- Hugely. Retrofitting cavity wall insulation can drop whole-house heat loss by 20–30%. Loft insulation (100→300 mm) drops it another 5–10%. Glazing upgrades matter less than people assume (~3–7%). Always model insulation first — it shrinks your heat pump kW, upfront cost, and running cost simultaneously. Put numbers on it with the [insulation savings calculator, and if you're a landlord with a MEES deadline, feed the result into the [MEES landlord compliance calculator.
Using this alongside other tools
Most people use this calculator twice — once for their current home to get a size estimate, then again after modelling insulation upgrades in the insulation savings calculator . Dropping cavity U from 1.5 to 0.45 alone typically cuts whole-house loss by 20–30%, often moving you from "needs a 9 kW heat pump" to "a 7 kW will do" — which directly lowers upfront cost.
When you're ready to compare running costs to your current boiler, move to the heat pump vs gas boiler calculator — and check grant eligibility in the Boiler Upgrade Scheme calculator .
Related calculators
Ranked by topic overlap with this tool.
- Heat Pump vs Gas Boiler Calculator UK — Compare running costs and 15-year total cost of ownership against keeping a gas boiler.
- Boiler Upgrade Scheme Grant Calculator UK — Check eligibility for the £7,500 BUS heat pump grant and see your net install cost.
- MEES Landlord Compliance Calculator UK — Landlord-focused MEES compliance check with cheapest-first measure stack and grant offsets.
- EPC Band Upgrade Cost Calculator UK — Cost of raising your EPC to band C, B or A — measure stack, grants, resale uplift and green-mortgage saving.
- Holiday-Let EPC Upgrade Calculator UK — Holiday-let EPC upgrade cost, occupancy-weighted running-cost saving, comfort-premium revenue uplift, and FHL capital allowance notes.