What Size Heat Pump Do I Need? (UK Guide)

A heat pump's size is its heat output in kilowatts: how much heat it can push into your home on the coldest design day. UK domestic units commonly span roughly 4kW to 16kW, and the Ofgem-reported average installation is an 8kW unit (at an average pre-grant price around £12,500, January 2026).

The right number for your home is not read off a floor-area chart. Two houses with identical floor plans can need very different heat pumps: one is a 1930s semi with solid walls and original windows, the other a 2010s build with cavity insulation and double glazing throughout. What decides the size is heat loss: how fast your home leaks heat on a cold day, which the system must replace in real time.

This is a genuine difference from boiler culture. Gas boilers are routinely oversized because oversizing a boiler costs little. Oversizing a heat pump costs efficiency and money, which is why the sizing process is formal.

Every Boiler Upgrade Scheme installation includes an MCS heat loss survey: it is part of how MCS-certified installers design the system. The surveyor works room by room, measuring:

• Fabric: wall construction and insulation, loft insulation depth, floor type, window glazing and frames
• Volumes and areas: room dimensions, external wall area, window area
• Air changes: how draughty each room is
• Location: the local design temperature (a January night in Aberdeen is not a January night in Exeter)

The output is a heat loss figure for each room and for the house, in kW. The heat pump is sized to meet it, and each room's radiators are checked against its number at the planned flow temperature.

Practical advice: ask to see the survey results. A reputable installer will happily share the room-by-room numbers. A "survey" that took ten minutes and produced no paperwork is a guess wearing a clipboard, and both oversizing and undersizing trace back to exactly that.

The levers, roughly in order of impact for typical UK homes:

• Insulation: loft insulation depth, cavity versus solid walls and whether they are filled or lined, floor insulation. This is the big one, and improving it before the survey can genuinely reduce the heat pump size you need.
• Glazing: single, double or triple, and how much of the wall is window.
• Airtightness: draughts around doors, floors, chimneys and loft hatches.
• Size and shape: more external surface area means more loss; detached homes lose more than mid-terraces of the same floor area.
• Where you live: design temperatures get colder as you go north. The same house needs more output in Scotland or the North East than on the south coast, and the survey accounts for it.

The cheapest kilowatt is the one you stop losing: loft and cavity insulation are usually far cheaper than the next heat pump size up, and they keep paying on every bill afterwards.

A heat pump is most efficient sending cooler water through the heating circuit than a boiler does. Cooler water means each radiator gives off less heat, so to deliver the same room temperature the system needs more radiator surface.

That is why the survey checks every radiator against its room's heat loss at the design flow temperature. Typical outcomes:

• Most radiators pass: many UK homes had radiators oversized for their boilers, which now works in your favour.
• A few get upsized: commonly a couple of rooms get deeper (double or triple panel) radiators in the same footprint.
• Underfloor heating, where present, is ideal: a huge emitter area running at low temperature.

Radiator changes are one of the items that push installed prices toward the top of the typical £8,000-14,000 pre-grant range, alongside cylinder work in combi homes. They are also exactly where you should not cut corners: a system forced to run hot because radiators were skimped loses the efficiency you bought it for. The physics is in our how heat pumps work guide.

The heat pump heats your tap water via a hot water cylinder, sized to your household:

• 1-2 people: smaller cylinders cover daily showers comfortably
• 3-4 people: the standard family sizes; the most common replacement class
• 5+ or bath-heavy households: larger cylinders, sized for the worst morning, not the average one

Your installer will size it from occupancy and usage patterns. Two notes specific to the UK switch:

Combi homes: if you currently have a combi boiler, you have no cylinder, and the installation adds one, usually in an airing cupboard, utility space or loft. It is a real cost line and a space conversation to have early; see our boiler replacement guide.

Recovery behaviour: a heat pump reheats a cylinder more gently than a boiler, so the cylinder is sized with a margin and typically reheats on a schedule. Correctly sized, you will not notice; undersized, you will, every busy morning.

Undersized: the home struggles to reach temperature on the coldest days, backup heaters run, bills rise, and confidence in the technology evaporates. Undersizing usually comes from skipping the survey or "matching the boiler's kW", which measures the wrong thing.

Oversized: a heat pump that is too big for the house short-cycles: it blasts to temperature, shuts off, cools, restarts, over and over. Cycling wears the compressor and drags efficiency well below the SCOP you paid for. Oversizing usually comes from fear-based "better safe than sorry" specs, and you pay for it twice: more upfront for the bigger unit, more forever in lost efficiency.

Right-sized: the unit runs long, low and steady, modulating with the weather, at or near its design SCOP of 3.8-4.5.

The defence against both is the same: a real heat loss survey, written numbers, and an installer willing to explain why the proposed kW matches your house. When comparing quotes, treat a big spread in proposed unit sizes as a question to resolve, not a detail. Get up to 3 quotes and ask each installer to justify the size; the explanations are usually more informative than the prices.