What size heat pump does your house need?

This works out how much heat your home loses, the heat pump output that implies, and whether it would cost more or less to run than what you have now.

It shows you every step of the arithmetic, where each number came from, and when it was last checked. It is an estimate built from a dozen answers, not a survey — the limits are set out in full with your result.

1 Your house

Where is it? Insulation data, age bands and grants all differ by nation.

Postcode Only the first part is used, to look up how long your heating season is.

Nearest of these These are the weather stations MCS designs against. Pick whichever is closest to you.

Total floor area m² Every floor added together. A rough measurement is fine.

Floors

Type Shared walls lose no heat, so this matters more than it sounds.

When was it built? Sets the assumed insulation standard where you do not know it.

Walls If you are not sure, leave the default for your build year.

Loft insulation Depth between the joists. A joist is about 100 mm deep, for scale.

Windows

People living here Only affects hot water, not the size of the heat pump.

Heating now Condensing boilers have a plastic flue and a condensate pipe draining away. Oil and LPG are not here yet — those prices are not capped, so we have no dated source to quote.

A new gas boiler would cost you £ If your boiler is near the end anyway, this is the fairer comparison. We do not guess a figure.

Installer quote, if you have one £ We will show you where it sits against what grant-funded installs actually cost.

No email address, no sign-up. Nothing you type is sent anywhere.

2 Heat loss and size

—kW

design heat loss at —, pointing to a heat pump rated —

The darker a part of the house, the more of your heat leaves through it. Air leakage has no surface to shade, so it shows as the plumes above.

Where the heat actually goes, in watts lost per degree of temperature difference. Every bar is an area multiplied by how fast heat passes through it; the figures are in the table below.

The working

Each row is an area multiplied by how fast heat passes through it. Check any line with a calculator.
Element	Area, m²	U‑value	Loss, W/K
Heat lost per degree of temperature difference			— W/K
Multiplied by the design temperature difference			— kW

3 Would it be cheaper to run?

—

efficiency your house needs,
just to break even

against

—

median measured across
428 real UK installations

The shaded band is the middle half of what 428 monitored UK heat pumps actually achieved. The marker is what your house would need.

Yearly running cost

The line is what you pay now. A bar that stops short of it costs you less to run; a bar that crosses it costs you more.

Most calculators assume the efficiency printed on the manufacturer's label. Field monitoring found real installations came in below that label figure in — of homes, so this page uses the measured spread instead. It produces smaller savings than you will see elsewhere.

4 What it would cost

6 What this cannot tell you

This is a whole-house estimate. A real design needs a room-by-room calculation to BS EN 12831‑1:2017, which is what MCS requires before anyone installs anything.
It cannot size your radiators, check whether your pipework is wide enough, or tell you if the outdoor unit will pass planning and noise rules.
It assumes a plain rectangular house. Extensions, conservatories, odd shapes and large amounts of glazing all move the answer.
Nobody can give you a fixed price without visiting. Treat this as a way of checking whether a quote is sensible, not as a quote.

The sensible next step is to have an MCS-certified installer do the full calculation on your property, and to get more than one quote.

7 Method and sources

How the heat loss is worked out

Areas are derived from your floor area, number of floors and house type, assuming a rectangular plan one and a half times as long as it is wide. Walls shared with a neighbour are excluded.
Each element's area is multiplied by its U‑value — how many watts pass through a square metre for each degree of temperature difference.
Air leaking in and out is added: 0.33 watt-hours per cubic metre per degree, multiplied by the air changes per hour and the volume of the house.
The total is multiplied by the difference between 21°C indoors and the coldest design temperature for your region. That gives the design heat loss in kilowatts.
Yearly heat is the same total multiplied by regional heating degree days. Hot water is added separately from the number of people.

Every number this page uses

What	Value	Source

Known gaps

These are not yet traced to a published source, and are flagged here rather than hidden: