What Size Heat Pump Do I Need?

Understanding how heat pump sizing works and why it matters.

Choosing the right size heat pump is one of the most important decisions in any installation. Too small and it may struggle to keep your home warm on cold days. Too large and it can become inefficient, more expensive, and may short cycle (turn on and off frequently).

Here’s what actually determines the correct size.

How Heat Pump Size Is Determined

Heat pump size is based on the **heat loss** of your property, not the size of your current boiler or the floor area alone.

A proper heat loss calculation takes into account:

The size and layout of the property
Insulation levels (walls, loft, floor)
Windows and doors
Air leakage and draughts
The type of heating system (radiators or underfloor heating)

This calculation tells the installer what size heat pump (in kW) is needed to keep your home comfortable on a cold winter day.

Common Mistakes in Sizing

Oversizing

Some installers oversize heat pumps to be “safe”. This can lead to higher upfront costs, inefficiency, and the system turning on and off too frequently (short cycling), which reduces its lifespan.

Undersizing

Undersizing usually happens when a proper heat loss calculation isn’t carried out. The heat pump may struggle to keep the home warm on very cold days.

What You Should Do

Ask any installer for a full room-by-room heat loss calculation.
Be cautious of anyone who sizes the heat pump based only on your current boiler size or floor area.
Understand that improving insulation can sometimes allow you to install a smaller (and cheaper) heat pump.
Don’t be afraid to get a second opinion if something doesn’t feel right.

Want to understand what size heat pump your home might need?

Use our Heat Pump Estimator to get a realistic indication based on your property.

Use the Heat Pump Estimator Try the Combined Estimator