Can solar panels cut the cost of heating a swimming pool?

Why does a heated pool cost so much to run?

Two reasons: there’s a lot of water, and it sits outside.

SPATA, the trade body for the UK pool industry, gives two example sizes in its energy-efficiency factsheet. A 6m by 3m above-ground pool at 1m deep holds somewhere around 18,000 litres. A 10m by 5m pool at 1.2m to 2m deep can be over 75,000 litres.

Warming water takes a fixed amount of energy. One kilowatt-hour lifts about 860 litres by a single degree, which follows from the heat capacity of water. So raising the small pool by 10 °C takes roughly 209 kWh of heat, and the large one about 872 kWh. That’s your one-off cost at the start of the season.

What runs the bill up is what happens afterwards. The pool leaks heat every hour it’s warmer than the air around it, and SPATA’s heating factsheet is blunt about where it goes: most of the loss is surface evaporation, with a smaller amount escaping through the shell walls and floor. Whatever you heat the water with, most of what you’re paying for is coming off the top of the water.

How much heat does an uncovered pool lose?

Enough that the sums are worth doing once.

Evaporating a litre of water takes about 0.68 kWh at pool temperatures — that’s the enthalpy of vaporisation — and the heat leaves with the vapour. SPATA’s water-losses factsheet offers 3mm in 24 hours as a guide for an uncovered indoor pool, and says outdoor pools can be significantly higher.

Take 3mm off the small pool’s 18 m² surface and you’ve lost 54 litres, which is about 37 kWh of heat, in a day. The large pool’s 50 m² gives you 150 litres and around 102 kWh. Since both come off the indoor guide figure, treat them as a floor rather than an answer. Sunshine on the water puts some of it back and the heater makes up the rest.

That’s how the covers factsheet can say an uncovered outdoor pool may lose “an incredible 90%” of its heat to evaporation. Next to the open surface, the shell and the pipework are rounding errors.

What’s the cheapest way to cut it?

A cover, before anything with a plug on it.

SPATA’s figure is that a heat-retaining cover can save as much as 80% of the heat lost from the pool surface, and its advice is to use one whatever your heat source. Run that through the numbers above and the small pool drops from roughly 37 kWh a day to about 7, the large pool from about 102 to about 20. You get that saving whether the heat comes from a roof, a heat pump or a gas boiler.

Two warnings from the same factsheets.

A safety cover is a different product. SPATA notes that safety covers are rarely the best at retaining heat but “do a great job of saving lives”, and suggests laying one over a heat-retention cover where safety is the priority.

Insulating the shell also matters far more indoors than out. Approved Document L sets a maximum U-value of 0.25 W/(m²·K) for a swimming pool basin in both its new-dwelling and existing-dwelling tables (4.1 and 4.2). Outdoor pools aren’t covered by building regulations at all. SPATA still thinks insulation is worth considering, with one caveat: the water may be colder at the start of the season because you lose the heat transfer from the ground.

Depth costs you too. SPATA points out that anything deeper than 1.2m leaves you heating a large volume of water that rarely gets used. A variable-speed filtration pump is on its list as well.

Which heater should the panels feed?

A heat pump built for pools, because it multiplies the electricity rather than simply converting it.

SPATA’s heating factsheet puts the multiplier at 3.5 to 4.5 times more heat into the water than the electrical input, across an outdoor pool’s heating season. That ratio is the CoP. A direct electric heater gives you one unit of heat per unit of electricity, and SPATA says its running costs are among the highest of any heat source going. Gas and oil boilers are out of the conversation here, since a roof can’t feed them.

Two things about pool heat pumps shape how they pair with panels.

The first is that they’re generally sized to match a pool’s average heat loss, not to haul it back up to temperature in an afternoon. So they run long, steady hours; SPATA warns that warm-up periods of under five days are generally unlikely. A slow daytime draw is the load shape a roof serves best.

The second is that CoP depends on the air. That 3.5–4.5 is an outdoor-season figure, and SPATA gives 2.5 to 3.5 as the winter average for an indoor pool’s unit. UK mean air temperature from May to September 2025 ran between 12.0 °C and 16.8 °C on the Met Office’s HadUK-Grid series, which is the range those seasonal figures are built on. When a quote gives you a CoP, ask which air and water temperatures it was measured at, and whether the test followed BS EN 17645:2022, the European standard for this equipment. SPATA’s own warning is that a lot of products from unknown sources are on the market, and that claimed performance should hold up in UK conditions.

How much of the pool’s electricity can a roof cover?

A useful share, and both sides can be written in the same unit.

On the European Commission’s PVGIS model (PVGIS-SARAH3, 2005–2023), a 4 kWp south-facing array at 35° with the default 14% system loss makes 2,194 kWh across May to September in Birmingham, 2,561 in Plymouth and 2,176 in Newcastle. Averaged over those 153 days, that’s 14 to 17 kWh a day.

Now the pool side, taking SPATA’s seasonal CoP at 4. Replacing the small pool’s evaporation at the indoor guide rate needs about 9 kWh of electricity a day uncovered, or about 2 covered. The large pool needs roughly 26 kWh a day uncovered and about 5 covered. The 10 °C lift at the start of the season works out at around 52 kWh of electricity for the small pool and 218 for the large one, so a few days of May output for one and a fortnight for the other.

On paper, then, a 4 kWp roof makes several times what a covered pool’s heat pump gets through on a summer day, and more than an uncovered small pool’s.

Three caveats on that. Outdoor evaporation runs above the indoor guide rate, so the pool figures are floors. The house is drawing on the same roof at the same time. And the roof’s day and the heat pump’s day aren’t the same shape, since a cloudy morning generates very little while the pump carries on regardless. Trust the seasonal and annual totals rather than any particular hour.

Does the timing actually line up?

Better than it does for anything else in the house.

PVGIS puts May to September at 57% of annual output in all three towns above. Birmingham’s month-by-month figures for the 4 kWp array run 149, 206, 336, 433, 469, 468, 472, 425, 360, 251, 169 and 133 kWh from January to December, so those five pool months beat the other seven combined.

Real meters show the same pattern. DESNZ’s Feed-in Tariff load factor analysis for 2024/25 puts the April-to-June quarter at 13.6% and October-to-December at 4.7%, close to three times the output per kWp in late spring as in late autumn. SPATA makes the same point from the pool’s end, noting that heat pumps tend to be most popular for outdoor pools because their operating months coincide with the warmest weather.

Timing matters for a second reason. Anything the roof makes that the house isn’t using at that moment gets exported. Under the Smart Export Guarantee, Ofgem’s rule is that generators are paid for the electricity they export, at a rate their supplier sets. Meanwhile every kWh you use at home is a kWh you haven’t bought at the unit rate. Put the pool’s heat pump on a timer that runs through daylight and you turn exported afternoon kWh into kWh the pool used. Same panels, different meter reading. Run it overnight and none of that applies.

Would solar thermal mats do it instead?

They can, and SPATA describes them without ranking them, so this guide does the same.

Its heating factsheet calls the idea attractive and says it will work for pool heating. It also says the UK climate makes it difficult to hold the pool at the temperature you want, so a supplementary heat source is often paired with the collectors. And it notes that a bank of standard pool collectors usually has to exceed 50% of the pool’s surface area, depending on location — so upwards of 9 m² for the small pool and 25 m² for the large one, run by a separate controller that only pumps water through the mats when there’s heat to collect.

The comparison worth making is about what the same area of roof does for the rest of the year. The Energy Saving Trust’s solar advice puts a 4.5 kWp electric array at 20 to 30 m² of roof, not far off the large pool’s collector bank. The collectors heat the pool in the months there’s sun to collect and do nothing in the others. The array feeds the pool’s heat pump in summer and the house for the remaining 43% of its year.

Which is the better use of the roof depends on whether you want the winter kWh, and whether you’re buying a heat pump anyway. If the pool already has one, the panels are the smaller addition.

What about an indoor pool?

The sums change, and not in the roof’s favour.

An indoor pool is heated year-round, and SPATA’s guidance is that the choice of heating will generally have to account for heating the air as well as the water, plus dehumidification, because some moisture will evaporate into the room whatever the cover does.

Its energy factsheet does note that in warm summer weather there’s little or no need to heat the pool hall, so summer generation still lands on a real load. But the winter half of an indoor pool’s year is exactly where the CoP falls to SPATA’s 2.5–3.5 and the roof’s output falls towards that 4.7% load factor for October to December. An indoor pool needs the most help in the months panels give the least. Winter is a job for the cover and the basin insulation. The roof handles the summer.

How does this site handle a pool?

Through the number you type in, rather than a pool question we don’t ask.

The usage step takes a monthly spend band, with a box underneath for a stated annual kWh figure that overrides the band completely. The sizing rule then works from that kWh.

If the pool’s heat pump runs off the house meter, its electricity is already inside the bill you’d read that figure from, and the roof card sizes to it without needing to know what the kWh went on. The model can’t tell a pool from a tumble dryer. If the pool has its own supply, add its annual kWh to the house’s before you type.

The roof card then measures your actual roof from imagery and prints its kWp and annual kWh side by side. How do installers turn kWp into kWh? shows the method behind an installer’s own estimate, and does where you live change what solar panels make? gives the zone figures for every postcode area if you want the paper number alongside the measured one.

What should I do first?

Cover, then heater, then roof, in that order.

The cover cuts the loss whatever your heat source, by up to 80% of the surface loss on SPATA’s figure, and it’s the only item on the list that makes every other item smaller.

The heater sets the multiplier: 3.5 to 4.5 across the season for a pool heat pump, against 1 for direct electric. Ask for a CoP tested to BS EN 17645, with the test temperatures attached.

Only then does the roof’s number mean much, because what it has to cover by that point is a covered pool’s heat-pump electricity. For a small pool that’s a couple of kWh on a summer day, against 14 or more from a 4 kWp array. Size the roof to a bill that already carries the pool, in the months the pool is actually on, and it stops being the load that wrecks your summer electricity costs.