Solar for Indoor vs Outdoor Swimming Pools
An outdoor pool is a seasonal sprint that happens to coincide with peak sunshine. An indoor pool is a marathon load that runs through the darkest week of December. They are both "pools," and they want close to opposite solar designs.
The outdoor pool: design for the season
Outdoor pool demand is a bell curve sitting almost perfectly under the solar production curve: heat-up in late April, maintenance heating through the summer, shutdown by October. This overlap means modest arrays achieve enormous demand coverage — the 1kWp-per-10m²-of-pool rule supplies a covered pool's heat pump for effectively the whole campaign, so most of the season's heat can be met without storage. The design priorities are afternoon-weighted orientation, a properly sized heat pump pairing, and ruthless attention to evaporation, since an uncovered outdoor pool loses heat to wind and sky faster than any sensible array can replace it. Where outdoor pools disappoint, the autopsy is almost always a missing cover or a heat pump scheduled to run at dawn.
What outdoor pools cannot expect from solar: shoulder-season miracles. Holding 28°C through a grey November requires buying most of the heat from the grid regardless of roof size — the honest options are accepting the season, or heating the human (a small heated splash of the budget goes a long way in wetsuit form for determined families).
The indoor pool: design for the year
An indoor pool hall is the most energy-intensive room in any UK building that has one: 28–30°C water, air held a degree or two warmer to manage condensation, and — the load owners underestimate — dehumidification running constantly, because every kilogram of evaporated water must be removed mechanically before it rots the building. The Carbon Trust reports that up to 65% of the energy used in a typical swimming pool building goes on pool heating and hall ventilation, and we found no published benchmark for a domestic indoor pool, so model yours from its own bills. Few domestic roofs will cover a year-round pool hall load with panels. The design goal flips from "cover the demand" to "buy down the baseload": the largest array the roof carries, feeding pool plant that never stops wanting power, achieves self-consumption fractions other solar owners dream about — every generated kilowatt-hour displaces a bought one, year-round, no export compromise.
Indoor specifics worth engineering properly: heat-recovery dehumidification units return evaporation heat to the water and slash the biggest line item; pool-hall covers still matter indoors (evaporation is humidity, and humidity is the dehumidifier's electricity); and because demand persists in winter, indoor pools are the one pool case where a battery and a time-of-use tariff genuinely earn their keep alongside the array.
Same hardware, opposite priorities
Both pools want PV feeding efficient electric plant — the technologies do not change, as the PV vs thermal comparison explains. What changes is the optimisation target. Outdoor: small-to-medium array, maximum seasonal overlap, zero storage, cover non-negotiable. Indoor: maximum array, baseload displacement, storage worth modelling, heat-recovery plant before panels. Budgets and payback bands for both appear on the costs page, the worked example runs an outdoor case end to end, and commercial-scale indoor pools — hotels, swim schools, leisure centres — continue on the commercial page. Unsure which physics problem you own? A pool with a roof you can see from the kitchen is a marathon; tell us about it via the contact form.
Outdoor sprint vs indoor marathon
| Outdoor pool Seasonal, May–Sept | Indoor pool Year-round hall | |
|---|---|---|
| Annual energy demand | about 6,600 kWh (our 8×4m covered example) | not published for domestic pools |
| Solar coverage achievable | most of the season | a minority of the year (model it) |
| Right array size | 1kWp / 10m² of pool | Largest the roof takes |
| Battery worth modelling | ||
| Dehumidification load | None | Constant — heat-recovery plant first |
| Cover's job | Evaporation = heat loss | Evaporation = dehumidifier cost |
| Typical solar payback | depends on what you replace | depends on what you replace |
Sources and assumptions
Figures were checked on 20 October 2026. Where we found no UK source we say so, and where a number is our own assumption it is labelled.
- Carbon Trust, Good Practice Guide GPG390 (April 2005) and guide CTG009 (April 2008): up to 65% of a pool building's energy goes on pool heating and hall ventilation; covers cut total pool energy use by 10–30%.
- SPATA, swimming pool heating factsheet: heat pump COP of 3.5–4.5 for outdoor pools in season, 2.5–3.5 for indoor pools in winter; solar thermal collector area usually exceeds 50% of the pool surface. Undated.
- SPATA (Swimming Pool and Allied Trades Association), pool covers factsheet: an uncovered outdoor pool may lose as much as 90% of its heat through evaporation; a heat-retention cover can save up to 80% of heat loss from the surface. Undated.
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