Heat pump vs solar pool heating
On-demand heat versus free-but-fickle sunshine: how the two heating routes compare, and when to combine them.
A heat pump and solar heating solve the same problem — cold water — from opposite ends. The heat pump is an appliance: a small refrigeration unit that pulls heat out of the outside air and pushes it into the pool, on demand, the moment you switch it on. Solar heating is not a heater at all; it is plumbing. Pool water gets diverted through dark absorber panels on a roof or a rack, picks up a few degrees from direct sun, and returns to the pool — no compressor, no thermostat, just physics doing what physics does whenever the sun is actually out.
What does not change between them is everything upstream and downstream. Both tap into the pool's existing circulation pump rather than replacing it, both need the water already balanced and turning over properly, and neither one earns its keep without a good cover — heating water overnight only to let it evaporate by morning is the definition of wasted energy. What changes is control: a heat pump holds a number you choose; solar hands you whatever the day offers.
- heat pump efficiency vs. direct electric heating
- 4–6×heat pump efficiency vs. direct electric heating
- heat pump sizing rule of thumb, covered pool
- 0.3–0.4 kW/m³heat pump sizing rule of thumb, covered pool
- collector area needed, as a share of the pool's surface
- 50–100%collector area needed, as a share of the pool's surface
- heating energy a nightly cover saves — before you even pick a heater
- 50–70%heating energy a nightly cover saves — before you even pick a heater
At a glance
| Heat pump | Solar absorbers | |
|---|---|---|
| Availability | On demand, any weather above ~5 °C | Only when the sun shines |
| Running cost | Electricity at 4–6× efficiency | Near zero |
| Upfront cost | Moderate | Moderate, needs 50–100% of pool area |
| Season stretch | Reliable spring–autumn | Helps shoulders, can't guarantee |
| Space needed | A washing-machine-sized unit | Large roof or ground area |
How each one actually heats the water
The heat pump works like a fridge in reverse. A fan pulls in outside air, a refrigerant absorbs its warmth and gets compressed until that warmth concentrates into something hot enough to hand to the pool water through a heat exchanger, and the now-cold air blows back out. For every unit of electricity spent running the compressor, the pump moves four to six units of heat — the whole reason it has largely replaced electric resistance heaters for pools.
A solar absorber has no machinery worth mentioning. Dark EPDM or polypropylene panels sit in full sun; a valve — usually triggered automatically once the panels are warmer than the pool — diverts a share of the pump's normal flow through them on the way back to the pool. Water picks up a few degrees crossing the panel and loses them again fast once the sun drops or a cloud rolls over. There is nothing to switch on: it works exactly as well as the weather that hour allows, and not a degree better.
Sizing, and where each one lives
The heat pump itself is a box roughly the size of a washing machine or an outdoor air-conditioning unit, sitting on a pad near the plant room with clear air on at least two sides — box it in and it starves itself of the air it needs to work. Sizing follows a simple rule of thumb: about 0.3–0.4 kW of capacity per cubic metre of covered pool water in a temperate climate, which puts a typical 8 × 4 m family pool around 12–16 kW — the free designer suggests a size from your own pool's dimensions rather than a generic one. It runs a fan continuously while heating, so site it away from a bedroom window or a neighbour's terrace, not tucked against the house wall closest to either.
Solar has no single unit to place — instead it is an array, and the array is the whole decision. Budget somewhere between half and the full surface area of the pool in collector panels for solar to do real work: an 8 × 4 m pool, at 32 m² of water, wants on the order of 16–32 m² of unshaded, well-oriented roof or ground rack. That is a serious claim on roof space, often competing with a future solar-electric array, and it needs deciding at the same time as the pool, not bolted on as an afterthought once the roof is already spoken for.
Running cost and reliability
The heat pump's running cost is real, but it is a fraction of what you would pay to heat the same water with a plain electric element, thanks to that four-to-six-times multiplier. It is also entirely reliable: switch it on in the morning and it keeps working through a grey afternoon or a cool evening, in effectively any weather down to around 5 °C outside air, which covers the vast majority of a European swimming season.
Solar's running cost is close to nothing — there is no compressor drawing current, just the pump doing a little extra work moving water through the panels. The trade is that supply is entirely the sky's decision. A run of overcast days is a run of days where solar simply does not contribute, and there is no override switch: it cannot promise a number for Saturday, only make the water somewhat less cold than it would otherwise be by the time Saturday arrives.
Season length and hitting a target temperature
If the goal is a specific number, held there reliably — 26 °C for serious laps, 28 °C for grandparents who won't get in below it — that is a heat pump's job, and only a heat pump's. It reaches a setpoint and defends it through the changeable weeks either side of summer, which is where a surprising share of a pool season's actually-swimmable days live.
Solar's real strength is exactly those shoulder weeks — spring and autumn, when a pool sits half-used because the water's a few degrees short of comfortable. On a sunny April or October day it adds real degrees for free; on a wet week in the same month it adds nothing, and there's no way to know in advance which you'll get. Paired with a heat pump, that's a genuine job well done: solar carries the easy, sunny days and the pump only has to step in on the rest. Run alone, most of Europe's climate leaves it a bonus rather than a plan — it stretches a good year and does very little for a bad one.
Where the money goes
Upfront, the two land in a similar order of magnitude for a typical family pool, but the money buys different things. A heat pump's cost is mostly the unit itself plus a straightforward electrical circuit run to the plant room — one purchase, one install day. Solar's cost is spread across the collector area, the mounting hardware for a roof or a rack, and the extra plumbing and valve; the bigger the array needs to be, the more of that budget is hardware rather than a single box.
After the install, the two costs move in opposite directions. The heat pump keeps drawing electricity every season it runs, which is real money even at high efficiency. Solar's running cost stays near zero for its working life, which can make it the cheaper choice within a handful of seasons if the site is genuinely sunny and the array generously sized — it just cannot promise the same output every single year the way a heat pump can.
Choose heat pump if… choose solar if… (or both)
Choose the heat pump if you want a number the pool actually holds — a target temperature that doesn't care whether the week turns out sunny or grey — and a season that reliably opens in May and closes in September. It is also the only sensible single choice where there simply isn't roof or ground space for a collector array worth the name.
Choose solar if there is genuinely spare, unshaded roof or garden to give it, the electricity bill matters more than guaranteed reliability, and a few free degrees on the days the sun cooperates is enough. Better than choosing, where the budget allows: run both, let the heat pump hold the baseline and solar take the load off it on bright days — and either way, put the first money into a good night cover, not the heater. A cover buys back most of the heat both of these machines exist to add, for a fraction of what either one costs.
The heat pump is the backbone: it guarantees the season. Solar is the bonus that trims its bill when the sun cooperates. If you can only do one, do the heat pump — and put the solar money into a good cover first.
Frequently asked questions
- What size heat pump does a typical family pool need?
- As a rule of thumb, 0.3–0.4 kW per m³ of covered pool water in a temperate climate — for a typical 8 × 4 m pool around 1.2–1.6 m deep, that lands near 12–16 kW. An undersized unit runs constantly and still loses the shoulder season.
- Can solar heating fully replace a heat pump?
- Only with a generous, unshaded, well-oriented collector area — 50–100% of the pool's own surface — and only through the sunniest months. Most gardens don't have that much spare roof, and no amount of panel helps through a cloudy week.
- Does a heat pump still work in cool or rainy weather?
- Yes — that is the entire point of it. It keeps producing heat in effectively any weather down to around 5 °C outside air, rain included, which is exactly when solar contributes nothing.
- Can solar panels be added to an existing heat pump setup later?
- Yes, easily. Solar taps into the same circulation loop through a diverter valve and a small controller, with no change needed to the heat pump itself — most solar installs are a top-up to an existing pump, not a standalone start.
- Do I still need a cover if I already have a heat pump?
- If anything, more so — an uncovered pool loses most of its heat to evaporation overnight, so heating water you then let evaporate away is paying for the same heat twice. Cover first, heat second, whichever heater is doing the work.
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