Heat pumps, honestly: what a garden pool costs to run

With a well-sized heat pump and a cover that goes on every evening, a fifteen-metre pool costs far less to keep warm than most people expect.

Running costs are the question we are asked most often, usually quietly and right at the end of the first visit. It is a fair thing to worry about. Twenty years ago an oil-fired pool could easily cost several thousand pounds a summer to heat. A modern pool, built and covered properly, is a very different thing.

How a heat pump does it

An air source heat pump takes warmth from the outside air and moves it into the water. For every unit of electricity it uses, it puts around four or five units of heat into the pool on a mild day. That is why we size them generously: a larger pump running gently is quieter and more efficient than a small one working flat out.

We tuck the unit into a planted enclosure or a plant room, well away from the terrace, so on a still evening you hear the birds rather than the fan.

The cover matters more than the heater

Most of the heat a pool loses goes straight up into the night air. An automatic cover, closed every evening and whenever nobody is swimming, cuts that loss by well over half. It also keeps leaves out and the water clean, so the filtration has less to do. If there is one thing we would never leave out of a budget, it is the cover.

What it costs in practice

The figures below come from the meters on pools we look after, swum from early May to the end of September and kept at 28 degrees. Your own costs will depend on the garden, the shelter and your tariff, and we will give you a proper estimate before anything is built.