What Size Heat Pump Do I Need?

It’s the first number anyone wants: how many kilowatts? And it’s the number most likely to be answered badly, because the honest reply is that nobody can tell you over the phone. Heat pump sizing is a calculation, not a rule of thumb — and getting it wrong is the single most common reason a system disappoints its owner.

Your boiler’s kW rating tells you nothing

Plenty of homes run a 24kW or 30kW combi boiler. That does not mean you need a 24kW heat pump. Boilers are routinely specified far larger than the house needs, partly because the extra capacity is cheap and partly because a combi needs raw power for instant hot water rather than for heating the building. Matching a heat pump to that figure would land you with something two or three times oversized.

What actually matters is heat loss: how quickly your house sheds warmth to the outside on a cold day. That’s a property of the building, not of whatever is currently bolted to the wall.

How a proper heat loss calculation works

A qualified surveyor works through the house room by room. Every external wall, window, door, floor and ceiling gets measured, and each is assigned a rate at which it loses heat based on its construction — solid brick behaves very differently to a filled cavity, single glazing very differently to modern double. Add an allowance for ventilation and draughts, set the outdoor design temperature for your area, and you get a figure in watts for each room and a total for the property.

That total is your heat pump size. It’s also the basis for radiator sizing and pipework decisions, which is why the same survey answers several questions at once. Under MCS rules this calculation isn’t optional — it’s a requirement of a compliant installation, and it’s part of what stands behind the Boiler Upgrade Scheme grant of £7,500 (£9,000 for homes coming off oil or LPG).

Most UK family homes come out considerably lower than people expect. Better-insulated modern houses often need less heat than a poorly insulated home of half the floor area, which is why two neighbours with identical footprints can end up with quite different units.

Why oversizing is the expensive mistake

There’s a tempting logic to fitting something bigger “just in case”. It backfires.

A heat pump is at its most efficient running gently and continuously at low flow temperatures. An oversized unit meets the house’s demand almost instantly, shuts off, cools, and fires up again — a pattern called short cycling. Each cycle is inefficient, and repeated over a heating season it drags your seasonal efficiency down. A well-designed system typically achieves a SCOP somewhere between 2.5 and 4; short cycling pushes you towards the bottom of that range or below, which shows up directly on your bill at roughly 24p per kWh of grid electricity.

Oversizing also costs more up front, needs a physically larger outdoor unit, and generally runs louder — which matters for the permitted development noise rules.

Undersizing has the opposite problem: on the coldest days the system can’t keep up and the backup immersion heater fills the gap using direct electricity, which is the most expensive heat you can buy. Both failure modes come from skipping the calculation.

The things that change the answer

Insulation is the biggest lever. Loft insulation topped up to current standards, cavity walls filled, draughts sealed — each reduces heat loss and therefore the size of unit you need, sometimes enough to move you down a model. If you’re planning insulation work anyway, do it before the survey rather than after.

Hot water demand matters too. The unit has to cover both space heating and reheating the cylinder, so household size and bathroom count feed into the specification. And your radiators are part of the system: existing ones may be perfectly adequate at lower flow temperatures, or a handful may need upsizing.

What to do next

If someone quotes you a heat pump size without measuring your house, treat that as a warning sign rather than efficiency. The calculation takes a proper survey, and it’s the difference between a system that quietly does its job for decades and one you fight with every winter.

Our survey is free and there’s no obligation — we’ll do the heat loss calculation, tell you the size your home genuinely needs, and be straight with you if a heat pump isn’t the right fit yet.