For hot water only, solar thermal wins where sunshine and roof space allow: collectors convert 60–70% of sunlight into heat and typically cover 50–70% of annual demand at zero fuel cost. A dedicated hot water heat pump wins where the roof is poor or demand is constant year-round, covering 100% of demand at a COP around 3. The lowest lifetime cost often combines both.
What does each technology actually deliver for hot water?
Solar thermal delivers a large share of your hot water for free but not all of it; a hot water heat pump delivers all of it, but every litre costs electricity. A correctly sized collector system reaches an annual solar fraction of 50–70% in most of Europe — around 90% of demand in summer but roughly 25% in mid-winter for central and northern Europe (Energy Saving Trust), with far flatter seasonal profiles in Turkey, Spain and the Mediterranean. A dedicated domestic-hot-water heat pump — a small air-source unit mounted on or beside its own cylinder, such as Solimpeks' Solido series with a 200 L or 300 L integrated tank and A+ energy label — extracts heat from ambient air at a COP around 3, day or night, January or July. So the real question is not which technology is "better" but which profile matches your building, roof and climate.
Which is cheaper to run for hot water?
Solar heat is free after installation; heat pump heat costs roughly one-third of the electricity price per kWh, because a COP of 3 turns one unit of electricity into three units of heat. Over a 20–25-year collector lifetime that difference compounds: solar thermal systems routinely outlast two generations of electrical appliances, while a heat pump water heater keeps drawing (cheap) power for every tank reheat. Upfront, the heat pump unit is usually the cheaper single purchase since it needs no roof work; solar thermal carries higher installation cost but qualifies for support in several markets, such as Ireland's €1,200 SEAI solar water heating grant and the UK's 0% VAT window until March 2027. Run the numbers for your own tariff with what is the payback period of a solar water heater.
When does solar thermal win?
Solar thermal wins when you have an unshaded, roughly south-facing roof and reasonable irradiation — conditions met across southern and central Europe. Collectors convert 60–70% of incident sunlight into useful heat, versus around 20% for PV panels making electricity, so per square metre of roof nothing beats thermal for hot water. In frost-light climates a pumpless thermosiphon system like the Solimpeks TSM range (120–305 L/day capacities) is the cheapest reliable hot water machine available — no controller, no pump, no compressor to service. Households with high daytime or summer-weighted demand (gardens, pools, holiday occupancy) also favour solar. What winter coverage really looks like month by month is detailed in how much hot water solar covers in summer vs winter.
When does a hot water heat pump win?
A hot water heat pump wins when the roof is shaded, occupied, structurally awkward or simply not yours — flats, north-facing terraces, listed buildings — and when demand is constant every week of the year. It needs only a power socket and an air path, covers 100% of demand with no backup heater in normal operation, and pairs naturally with cheap overnight or midday-solar electricity tariffs — see which electricity tariff is cheapest for a heat pump.
| Factor | Solar thermal | Hot water heat pump |
|---|---|---|
| Annual coverage | 50–70% (rest needs backup) | 100% |
| Running cost | Zero fuel | Electricity ÷ COP ≈ 3 |
| Roof required | Yes, unshaded | No |
| Winter output | Low (≈25% coverage in central Europe) | Full, at reduced COP |
| Typical service life | 20–25+ years | 12–15 years |
| Best fit | Sunny climates, daytime/summer demand | Flats, shaded roofs, constant demand |
Can I combine solar thermal with a heat pump?
Yes — a twin-coil cylinder with solar on the lower coil and the heat pump on the upper is the classic zero-conflict pairing: solar preheats whenever the sun allows, and the heat pump only tops up what solar left undone. That cuts the heat pump's workload by the solar fraction and extends its life through fewer cycles. The piping logic is covered in how do I combine solar thermal and a heat pump on one cylinder. If your decision also involves space heating — not just hot water — the three-way comparison in solar thermal vs PV + diverter vs heat pump is the right page.
Frequently asked questions
Is a heat pump or solar thermal cheaper for heating water?
Per kWh of heat, solar thermal is cheapest — the fuel is free — but it covers only 50–70% of annual demand, so you still pay for backup heat. A hot water heat pump covers everything at roughly one-third of the electricity price per kWh. Total cost depends on your climate, roof and tariff.
Can solar thermal supply all my hot water?
Not year-round in most of Europe. Coverage reaches about 90% in summer but falls to roughly 25% in mid-winter in central Europe, giving an annual solar fraction of 50–70%. In high-irradiation regions like southern Turkey or Spain, annual coverage runs higher but still needs winter backup.
Do hot water heat pumps work in winter?
Yes. Air-source hot water heat pumps keep working in cold weather at a reduced COP, and units drawing from indoor or plant-room air barely notice the season. Unlike solar thermal, output does not depend on sunshine at all.
What lasts longer, a solar water heater or a heat pump water heater?
Solar thermal collectors routinely last 20–25 years or more — Solimpeks designs its collectors for a 20–25-year service life — while heat pump water heaters, containing a compressor and fan, typically last 12–15 years. Over 25 years you may buy one solar system but two heat pumps.