Heat Loss Calculator

Heat loss is the single number that determines what heating system your home needs. It is how fast your home leaks heat to the outside on a cold day, measured in kilowatts, and any boiler or heat pump has to be able to replace it at least as fast.

Enter your floor area and pick the closest match for your walls and windows. The calculator estimates your heat loss at design conditions and suggests the heat pump output to start looking at.

What is heat loss?

Every home loses heat through its walls, roof, floor, windows and through air leaking in and out. The colder it is outside, the faster that happens. Heat loss is the rate of that leak at a chosen design condition, and it is expressed in kilowatts because it is a rate rather than a quantity.

This matters because heating systems are sized in kilowatts too. If your home loses 6 kW on a cold day, a 5 kW heat pump cannot hold temperature no matter how long it runs.

How this calculator works

The calculator uses a heat loss parameter: a typical number of watts lost per square metre of floor area, for each degree of difference between inside and outside. That parameter changes with construction, so the calculator varies it by wall type and adjusts it for glazing.

The estimate is then:

floor area × heat loss parameter × temperature difference

The temperature difference is your target indoor temperature minus a design outdoor temperature of −3 °C, which is the level UK heating design commonly assumes. At a 21 °C target that gives a 24 °C difference.

Why heat pump sizing matters

Sizing errors go wrong in both directions, but not symmetrically. An undersized heat pump cannot keep the house warm on the coldest days, which is the failure people notice. An oversized one costs more to buy and cycles on and off rather than running steadily, which is less comfortable and less efficient.

Because being short is the worse outcome, this calculator rounds its suggestion up to the next half kilowatt rather than to the nearest.

What this cannot tell you

This is a whole-house benchmark, not a survey. It does not know how much of your wall is window, whether your loft insulation is at current depth, how draughty the house is, or how any individual room behaves. Two homes with identical floor areas and wall types can differ substantially on all four.

Before anyone specifies equipment, an MCS-certified installer must carry out a room-by-room calculation. Treat this figure as a way of knowing roughly what to expect and whether a quote looks sensible, not as a specification.

If you want to know what a heat pump would cost to run rather than what size it needs to be, the solar and heat pump data page carries observed UK deployment figures.