Solar PV converts sunlight into electricity at around 20% efficiency; solar thermal converts it into heat at 60–70%. That means a thermal collector harvests roughly three times more energy per square metre of roof when the end use is hot water, while PV produces electricity that can power anything, be stored in a battery or exported to the grid. They are complementary technologies, not competitors for the same job.
What does each technology actually do?
Solar PV (photovoltaic) panels use semiconductor cells to convert light directly into electricity — a universal energy carrier that can run appliances, charge a car or be sold to the grid. Solar thermal collectors use a dark absorber plate to convert sunlight into heat, carried by a fluid to a hot water cylinder. One makes electrons, the other makes hot water; the confusion arises only because both sit on roofs and both are called "solar panels" in everyday speech. A third category, the PVT hybrid collector, combines both functions in a single panel.
Why does solar thermal capture more energy per square metre?
Because heat is thermodynamically easier to make than electricity. A PV cell can only convert photons within a specific energy band, and around 80% of the incoming solar energy is lost as unused spectrum and cell heating. A thermal absorber simply soaks up almost the entire spectrum as heat, losing energy only through the collector's glazing and insulation. The result at water heating temperatures: 60–70% conversion efficiency for a good flat-plate collector against roughly 20% for a PV module. Per square metre and per year, that translates into a large heat harvest — the actual kWh figures for European climates are worked through in how much heat one square metre of collector produces.
Where each technology wins
| Criterion | Solar thermal | Solar PV |
|---|---|---|
| Energy form | Heat (water at 40–90 °C) | Electricity |
| Conversion efficiency | 60–70% at DHW temperatures | ≈ 20% |
| Energy per m² of roof (hot water duty) | Highest | Lower |
| Flexibility of use | Hot water and heating support only | Any electrical load, export, batteries |
| Storage | Insulated water cylinder — cheap, simple | Battery — effective but costly per kWh |
| Typical system size | 4–6 m² for a family home | 10–30 m² |
| Best-fit load | Constant hot water demand | Daytime electrical demand or export tariff |
The pattern is clear: when the target is specifically hot water and roof space is limited, thermal collectors deliver more of it per square metre. When the target is general energy supply and the roof is large, PV's flexibility dominates.
Can PV make hot water too?
Yes — via a power diverter feeding an immersion heater, or by running a heat pump — and this is where the real 2026 buying decision sits. Diverting surplus PV into a cylinder turns ~20%-efficient electricity back into heat, which is thermodynamically wasteful but uses electricity that would otherwise be exported cheaply. Pairing PV with a heat pump multiplies each diverted kilowatt-hour by the heat pump's COP. Comparing these routes properly is a purchasing question, not a physics question, and it has its own dedicated page: solar thermal vs PV + diverter vs PV + heat pump.
So which is "better"?
Neither, in the abstract — they answer different questions. Solar thermal is the specialist: unmatched heat yield per square metre, simple cylinder storage, proven 20-25-year hardware, which is why manufacturers like Solimpeks have been building collectors continuously since 2001. PV is the generalist: universal output and falling hardware prices. The claim that cheap PV has made solar thermal obsolete deserves — and gets — an honest, numbers-based treatment in is solar thermal dead now that PV is cheap?. And where roof area must serve both demands at once, PVT hybrid panels produce electricity and hot water from the same square metre.
Frequently asked questions
Can I have both solar PV and solar thermal on the same roof?
Yes, and many households do: a small thermal array (4–6 m²) for hot water plus PV on the remaining area. They use separate circuits and do not interact. PVT hybrid panels are the single-field alternative when space is tight.
Which is cheaper to install, PV or solar thermal?
A domestic solar thermal system is usually the smaller investment because the array is smaller, but PV benefits from mass-market hardware pricing. Compare cost per useful kilowatt-hour for your own hot water and electricity demand rather than headline system prices.
Do solar thermal collectors work at night or in winter?
Not at night — there is no radiation to harvest, so the insulated cylinder carries hot water through to morning. In winter output falls substantially but does not stop; collectors still harvest direct and diffuse radiation on cold, clear days.
Is solar thermal or PV better for a swimming pool?
Solar thermal, decisively. A pool needs large volumes of low-temperature heat, which is exactly where collector efficiency peaks. Producing that heat via PV and electric heating would need several times the roof area.