What size heat pump do you need, and where does the number come from?

A heat pump is sized to one question: on the coldest day your area gets, how fast does your house lose heat? Match the output to that loss and you are done. Everything else is detail.

This page shows the whole calculation on a real example, so you can follow it with a calculator. If you just want the answer for your own house, use the size and cost calculator.

1 The whole thing in one line

Heat loss per degree, multiplied by how many degrees colder it is outside, gives the kilowatts your house needs. That is the size of the heat pump.

Heat loss per degree is itself two things added together: heat escaping through walls, roof, floor and windows, and heat carried out by air leaking through the building. Both are measured in watts per degree of temperature difference.

2 A worked example

Step one: how much of the house is exposed

The example house, shaded by where its heat goes. Walls dominate, which is why wall insulation changes the answer more than anything else you can do.

Step two: heat loss through each surface

Area multiplied by U‑value. A U‑value is how many watts pass through a square metre for each degree of difference, so a lower number is better.
Element	Area, m²	U‑value	Loss, W/K
Heat lost per degree of temperature difference			— W/K
Multiplied by the design temperature difference			— kW

Step three: pick a unit

3 Why floor area alone gets it wrong

Plenty of calculators ask only for bedrooms or floor area. Here is the same — m² house, identical in size, with one thing changed at a time.

Change	Design heat loss	Unit

4 Bigger is not safer

Too small and the house never reaches temperature on the coldest days, so the immersion heater or backup element runs and your bills climb.
Too large and the unit short cycles, switching on and off instead of running steadily. That wears it out and drops its efficiency, which is the one thing that determines whether it is cheaper to run than gas.
Oversizing is the more common mistake, and it is expensive twice: a bigger unit costs more to buy and then runs worse.

5 What an estimate cannot do

MCS requires a room-by-room calculation to BS EN 12831‑1:2017 before anyone installs anything. That works out the loss of every room separately, so each radiator can be sized. A whole-house estimate cannot do that.
Air leakage is assumed from the age of the property here. In a real survey it is measured. It is typically 15–30% of the total, so it is the biggest single source of uncertainty in any estimate.
Nobody can give you a fixed price or a final size without visiting.

Use an estimate to sanity-check a quote, not to replace a survey. If an installer's figure is far from what you work out here, that is a question worth asking, not proof that either of you is wrong.