The Cheapest Way to Heat a Swimming Pool in the UK

The short answer

The cheapest kilowatt-hour of pool heat is the one you never buy, so the cheapest way to heat a swimming pool in the UK starts with a cover, not a heater. US Department of Energy guidance says evaporation is by far the largest source of pool energy loss and credits covers with saving about 50% to 70% of pool heating costs; the UK trade body SPATA says an uncovered outdoor pool may lose as much as 90% of its heat through evaporation. After the cover comes the heat source, and in 2026 the ranking on running cost is: a heat pump fed by your own solar panels, then a heat pump on grid electricity, then a gas boiler, then direct electric heating. Solar thermal mats have no fuel cost at all but only work when the sun does.

What a kilowatt-hour of pool heat costs in 2026

Fuel prices are quoted per kilowatt-hour of fuel, but a pool needs kilowatt-hours of heat. A boiler wastes some of its fuel; a heat pump multiplies its electricity. Dividing the price by the efficiency (or the heat pump's coefficient of performance, COP) gives the number that matters: pence per kilowatt-hour of heat delivered to the water. The table uses Ofgem's price cap for 1 October to 31 December 2026: electricity 26.32p per kWh (no VAT in this period) and gas 7.97p per kWh including 5% VAT.

Three things stand out. First, the gap between a heat pump and a gas boiler is smaller than brochures suggest: at a seasonal COP of 3 the two cost about the same, and the heat pump only pulls clearly ahead at COP 4 and above. Second, direct electric heating is nearly three times the cost of gas per unit of heat. Third, solar panels change the sum rather than the ranking: electricity you generate and use yourself costs you the export payment you did not receive, which is 2–6p per kWh on standalone tariffs and 5.6–16p where you also buy your electricity from the same supplier (MoneySavingExpert, October 2026), so a heat pump running on your own generation turns that into roughly 0.5–4p per kWh of heat.

Why a cover beats every heater on this list

An outdoor pool loses most of its heat from the water surface, and most of that is evaporation. A cover stops it. Because every heat source in the table above has to replace what the pool loses, cutting the loss cuts the heating bill by the same proportion whichever heater you use. The worked example on this site assumes a bubble cover used overnight and on non-swimming days removes about 40% of the total heat demand of an 8×4 m pool, which is a cautious figure against the DOE's 50% to 70% heating-cost saving range.

On our 8×4 m example pool, heated to 28°C from May to September, that is 11,000 kWh of heat uncovered and about 6,600 kWh covered. At the gas price in the table, the 4,400 kWh difference is worth about £410 a season on gas (less, about £290, if the pool is heated by a heat pump). Plan on a few hundred pounds for a bubble cover and roller (prices vary by size and roller quality, so get a quote); even at the top of that range it repays itself in about two seasons, and at the bottom of it inside one.

Then a variable-speed pump

The circulation pump is the other quiet cost. Pump power falls with the cube of the flow rate (a standard pump affinity law), so running a variable-speed pump slowly for longer filters the same water for a fraction of the electricity. In the worked example a 1.1 kW single-speed pump running eight hours a day through a 153-day season uses about 1,350 kWh, about £355 at 26.32p; a variable-speed replacement doing the same job uses roughly 400 kWh, about £105. The saving is a heating-bill-sized number that has nothing to do with the heater, which is why it comes second.

Choosing the heat source once demand is down

If you already have a gas boiler

Keep it if it works and the pool is covered: at 9.4p per kWh of heat a covered 8×4 m pool costs about £620 to heat in a season on our assumptions. Replacing it with a heat pump saves money only if the seasonal COP is comfortably above 3.5 — the difference at COP 4 is roughly 2.8p per kWh, about £180 a season on 6,600 kWh. That is a slow payback on its own, which is why the heat pump case is strongest when it is paired with solar panels or replaces a boiler that is due for replacement anyway.

If you heat with direct electric

This is the case where a heat pump pays back fastest. At COP 4 the same heat costs about a quarter of what a direct electric heater charges, so the saving on 6,600 kWh is roughly £1,300 a season. Do the cover first and the heat pump you buy can be one size smaller.

If you have, or are planning, solar panels

PVGIS, the European Commission's free solar-yield tool, gives about 981 kWh per kWp a year for a south-facing 35° roof near Leicester, and about 56–58% of the year's output arrives between May and September across the UK locations we checked. A 3 kWp array therefore produces roughly 1,700 kWh in the swimming season, which is about what a covered 8×4 m pool's heat pump and variable-speed pump need on our assumptions. DESNZ's 2025/26 cost data puts the GB median for small installs at £1,595 per kW, so 3 kWp is about £4,800 — but if the panels are going on the house anyway, the extra cost of letting the pool use them is close to nothing.

If you only swim in summer and have a sunny roof

Solar thermal mats have no fuel cost and the lowest running cost of anything in the table. They suit the summer-only pool that just needs a few degrees on top of the sun's own heating. We compare them properly with PV on the PV vs solar thermal page, and price both on the costs page.

What does not make the list

Oil, LPG, biomass and electric immersion systems are not priced here because their prices vary by supplier and delivery, and we have no published GB average to cite. Pool dehumidification and indoor-pool air handling are a separate calculation, covered on the indoor vs outdoor page. For hotel, spa and leisure-operator pools the same ranking holds but the money is made on plant efficiency and tax treatment rather than the heat source alone; see the hotel and leisure pools page.

Sources and how the numbers were built

• Ofgem, Changes to energy price cap between 1 October and 31 December 2026, published 26 August 2026: electricity 26.32p/kWh (no VAT), gas 7.97p/kWh (5% VAT), GB average, Direct Debit.
• US Department of Energy, Better Buildings, Pool Efficiency: evaporation is by far the largest source of pool energy loss; covers can save about 50–70% of pool heating costs. The UK trade body SPATA (pool covers factsheet) says an uncovered outdoor pool may lose as much as 90% of its heat through evaporation.
• European Commission JRC, PVGIS v5.3, SARAH3, 35° tilt, due south, 14% system losses, queried 20 October 2026: Leicester 981, Leeds 947, Exeter 1,052 kWh/kWp/year; May–September share 56–58%.
• DESNZ, Solar PV cost data, 2025/26 edition (published 28 May 2026): GB median £1,595/kW for 0–4 kW installs.
• Smart Export Guarantee rates: MoneySavingExpert (updated 6 October 2026): standalone tariffs 2–6p/kWh; 5.6–16p/kWh where you also take your electricity from the supplier. Rates move.
• Our assumptions (shown, not sourced): 8×4 m pool, 11,000 kWh uncovered heat demand May–September at 28°C, bubble cover cutting total demand by 40%, 85% boiler efficiency, 1.1 kW single-speed pump for 8 hours a day.

Questions people ask

Is a heat pump cheaper than gas for heating a pool?

On running cost, a heat pump with a seasonal COP of 4 delivers heat at about 6.6p per kWh against about 9.4p for gas at 85% efficiency (Ofgem cap, October to December 2026). At a COP of 3 the two are about level. The heat pump wins more clearly when it runs on your own solar electricity.

What is the cheapest way to heat a swimming pool?

Stop the heat leaving first. A cover used whenever the pool is not in use cuts heat demand far more cheaply than any heater, and every heat source then has less to replace. After that, a heat pump (ideally fed by your own panels) is the cheapest way to supply the remaining heat.

Is solar thermal free to run?

There is no fuel bill, but the circulation pump has to push water through the roof circuit, so there is a small electricity cost. Solar thermal also only helps when the sun is out, so it extends the season by a few weeks rather than guaranteeing a temperature.