Solar PV makes electricity, solar thermal makes hot water.
Solar PV is far more versatile, you can heat water using an electric heater, and power anything in your home that uses electricity including an EV.
Solar thermal works great if you’re replacing an electric boiler and not so great if you’re replacing gas, as the costs don’t add up.
In 2025 the UK installed a record 267,032 certified solar PV systems. In the whole of 2024 it installed 180 solar thermal systems, the lowest figure since records began in 2009. Britain now fits more photovoltaic arrays in a single day than solar collectors in an entire year. This guide explains why, and identifies the narrow set of cases where the market has it wrong.
- The higher efficiency comparison makes for good marketing but doesn’t mean much in reality. Electricity powers everything; heat only heats water.
- Thermal wins per square metre of roof, PV wins per pound of capital invested. A collector delivers more energy per square metre, but costs roughly five times as much per square metre installed.
- Real-world thermal output falls short of the brochure. The median system in Energy Saving Trust field trials supplied 39% of annual hot water, against the 50-70% commonly marketed.
- On a gas-heated home the gap is five to one. £6,000 in solar thermal returns about £146 a year. In solar PV, about £729.
- Thermal still wins in one situation. Electric hot water plus a small or awkward roof.
They do different jobs
Solar photovoltaic panels convert light into electricity. Solar thermal collectors convert light into heat in a fluid. Both sit on a roof, that’s the only similarity.
Electricity is multi-use. It runs the kettle, the heat pump, the car and the immersion heater, and anything you cannot use can be sold to the grid. Heat is not. A solar collector produces hot water and nothing else, it cannot be exported or stored, and once the cylinder is full, it just sits there degrading the system.
That difference explains almost every finding below: one output is worth more than the other in a modern British home.
- Solar fraction
- The share of annual hot water demand a collector supplies. Often lower in practice than in modelling.
- Self-consumption
- The share of PV generation used in the house rather than exported. Batteries, EVs and diverters raise it.
- Smart Export Guarantee
- The scheme obliging suppliers to pay for exported electricity. Solar thermal has no equivalent.
- Diverter
- A device sending surplus PV electricity to the immersion element, letting panels do the collector’s job at low cost.
- Twin-coil cylinder
- A cylinder with two heat exchangers, required for solar thermal and not for PV.
- Stagnation
- Collector overheating when the cylinder is already hot. A thermal failure mode with no PV equivalent.
The efficiency trap
A good evacuated tube collector converts 50% to 70% of incoming solar radiation into heat. A modern photovoltaic panel converts 20% to 23% into electricity. But conversion efficiency measures how well a device does its own job, not how useful that job is. A kettle converts nearly 100% of its input into heat and a petrol car about 20% into motion, and nobody concludes that kettles are better vehicles.
The right question is how much money each square metre of roof, and each pound of capital, returns per year.
Head to head on twelve measures
| Measure | Solar PV | Solar thermal |
|---|---|---|
| Output | Electricity, any use | Heat, hot water only |
| Conversion efficiency | 20-23% | 50-70% |
| Yield per m² per year | ~180 kWh electric | ~300-450 kWh heat |
| Installed cost per m² | ~£286 | ~£1,418 |
| Typical system cost | £6,900-£9,200 | £4,200-£6,500 |
| Return on capital | 10-15% a year | 2-8% a year |
| Revenue side | SEG export income | None |
| Maintenance over 20 years | Low, likely one inverter replacement | £800-£1,400 |
| Service life | 25-30 years | 15-25 years |
| Needs a cylinder | No | Yes, twin-coil |
| Works with battery, EV, heat pump | Yes | No |
| UK installations, latest year | 267,032 (2025) | 180 (2024) |
Thermal takes two of the twelve rows, and they are the two that get quoted. The cost per square metre row decides the outcome, and it is the row nobody publishes.
Per square metre of roof
On a strict energy basis, solar thermal is the better use of a square metre. Four square metres of evacuated tube produces roughly 1,800 kWh of heat a year in modelling. Four square metres of PV, about 0.85 kWp, produces roughly 720 kWh of electricity. Convert both to money and the ranking depends entirely on what the heat is replacing.
Against an electric immersion, thermal returns nearly three times what PV does. Against mains gas, they are within a couple of pounds, and a household with a battery or an electric car pulls ahead on PV.
Value per square metre only matters if square metres are the binding constraint. For most British houses the limit is budget, not roof.
Per pound of capital
A 4 m² solar thermal system at £5,670 works out at £1,418 per square metre installed. A 6 kWp PV array at £8,000 covers about 28 square metres, which is £286 per square metre. That gap swamps the energy advantage. Run the same £6,000 into each technology on a mains gas home:
1,800 kWh of heat displacing gas at 8.1p per kWh
4.5 kWp generating 3,825 kWh, half used and half exported
| Scenario | Annual return | Payback | Verdict |
|---|---|---|---|
| PV, EV household | £880 | ~9 yrs | Strong |
| PV, typical gas household | £729 | ~11 yrs | Strong |
| Thermal, electric hot water | £305-£470 | 12-19 yrs | Workable |
| Thermal, LPG | £232 | ~24 yrs | Marginal |
| Thermal, oil | £173 | ~33 yrs | Weak |
| Thermal, mains gas | £95-£146 | 39-60 yrs | No |
Mains gas heats water in most British homes. On modelled output the payback is 39 years and on field-trial output it is nearer 60. Either figure exceeds the life of the collectors, so essentially pointless.
What thermal actually delivers
Manufacturers and installers typically quote a solar fraction of 50% to 70% of annual hot water. The Energy Saving Trust’s field trial of 88 UK and Irish systems found a median of 39%, ranging from 9% to 98%. Well-installed, properly used systems supplied around 60%.
That gap is not evidence of bad products. It reflects what modelling struggles with: cylinder standing losses, boilers set to fire before the sun has had a chance, hot water drawn in the morning rather than the evening, and controllers left on default settings.
The largest recoverable loss is boiler timing. If the boiler heats the cylinder at 6am, the collector spends the day with nowhere to put its heat. Set the backup to fire in the late afternoon, and ask installers for a measured solar fraction, not a modelled one.
Solar PV has no equivalent gap, because its output is measured at the meter. Where it disappoints, shading is almost always the cause, and it’s visible before you install during a survey.
The optionality argument
The strongest case for PV is not on any spec sheet. British homes are using more electric over time (due to EV use, and more technology in the home), and it’s costing more each year, which increases what a photovoltaic array is worth. The same doesn’t hold up as well for a solar collector, as people aren’t using more hot water, even if costs are higher.
- Batteries. Store surplus generation for the evening peak, lifting self-consumption from around 30% to 70%.
- Electric vehicles. Add roughly 2,100 kWh a year of shiftable demand that panels can serve.
- Heat pumps. Turn electricity into heating and hot water at a coefficient of performance of around three, competing directly with what the collector was for.
- Immersion diverters. Let panels do the collector’s job on roof space you already fitted.
- Export tariffs. Surplus PV typically earns a few pence to around 15p per kWh through the Smart Export Guarantee. Surplus heat earns nothing and causes excessive system wear.
A solar collector installed in 2014 does exactly what it did then. A PV array installed in 2014 is now worth more, because the household around it has acquired new ways to use what it makes.
What the market decided
Solar thermal installations peaked in 2011 and fell through most of the Renewable Heat Incentive years, apart from a one-off spike in 2021. Since the RHI closed, just over 900 systems were installed across 2022 and 2023 combined. PV went the other way with over 220,000 installs in 2025 . That should tell you everything.
The Feed-in Tariff drives PV to 203,125 installations in 2011, while solar thermal hits its own peak of 7,890.
Solar thermal gains a payment per kilowatt-hour of heat generated, yet annual installations keep falling.
The FiT closes to new applicants in March 2019 and the SEG starts in January 2020, keeping PV a revenue route through export payments.
Solar thermal loses its only subsidy and gains no replacement. Existing recipients keep their seven years of payments.
The lowest annual total since MCS records began in 2009, against about 44,000 systems since then.
267,032 certified PV installations in 2025, 31% above 2011, and almost 150,000 in the first half of 2026.
Which to choose, by situation
Scores are out of five and are our editorial judgement, weighted for return on capital, roof constraints and how the household is likely to change over the system’s life.
The commonest UK situation and the clearest answer. Gas delivers heat at around 8.1p per kilowatt-hour, so displacing it with a collector returns very little, while PV electricity displaces imports at 26.11p and exports the rest. A diverter handles summer hot water anyway.
Why: Five times the annual return for the same capital
Self-consumption rises towards 70 to 80%, pushing the value of each generated kilowatt-hour close to the full import price. A collector contributes nothing to either load and competes for the same roof.
Why: Every additional electrical load raises PV’s value and leaves thermal’s unchanged
Thermal returns more per square metre, but if roof area is not the constraint that does not matter. A larger PV array plus a diverter covers hot water in summer, everything else year-round, and earns export income.
Why: More total output when area is not limiting
The one domestic case where thermal wins outright. Below roughly ten usable square metres, four square metres of collector returns around £470 a year against about £188 from the same area in panels. Flats with electric cylinders, dormer roofs and heavily shaded properties with one clear patch all qualify.
Why: Two and a half times the return where square metres are the binding constraint
Guest houses, care homes, gyms, campsites and laundries have large steady loads that keep pace with summer output, so stagnation stops being a design limit. This is where solar thermal remains properly competitive rather than merely defensible.
Why: Demand matches supply across the whole season
A heat pump already makes hot water for roughly 9p per kilowatt-hour on the price cap, and less on a heat pump tariff, so a collector would be displacing heat that is already cheap. Panels feed the heat pump directly and cut the cost of everything it does.
Why: Thermal and heat pumps are alternatives for the same load, not partners
Can you have both?
Yes, and it is rarely the right answer. Three routes exist.
Side-by-side installation doubles the number of trades, certificates and points of failure, and the collectors take space that would have gone to panels.
Hybrid PVT panels generate electricity and capture waste heat from behind the cells in one unit. Attractive on a small roof, but cooling the cells means running the thermal loop at low temperature, so you get plenty of tepid water rather than a useful volume of hot water, at £550 to £850 per square metre.
The third route is the one most households should take. A PV array with an immersion diverter does the collector’s job at perhaps 60% of its effectiveness for £400 to £600, uses no additional roof, needs no glycol, no pump and no annual service, and reverts to normal PV behaviour the moment the cylinder is satisfied.
If you already have a working solar thermal system, keep it. The capital is spent and the running cost is a glycol service every five to seven years at £150 to £300. This comparison is about where to put new money.
Frequently asked questions
Which is better, solar panels or solar thermal?
Solar PV for the large majority of UK homes, because it returns more for the same money. The exception is a home with electric hot water and under about ten square metres of usable roof, where a collector wins.
Is solar thermal more efficient than solar PV?
Yes, at converting sunlight, and it is the wrong measure. Heat only heats water, while electricity does anything and can be sold. The useful comparisons are value per square metre of roof and return per pound of capital.
How much does each cost in the UK?
A 4 m² solar thermal system costs £4,200 to £6,500 installed including a cylinder. A 6 kWp PV array costs £6,900 to £9,200. Both are zero-rated for VAT until 31 March 2027.
What is the payback on each?
PV typically pays back in 9 to 11 years. Solar thermal pays back in 12 to 19 years against an electric immersion, around 24 years against LPG, 33 against oil, and 39 to 60 against mains gas.
Can solar panels heat my water instead?
Yes, using an immersion diverter costing £400 to £600. For most gas-heated homes this is the better route.
Is there a grant for either technology?
Not for most households. The Boiler Upgrade Scheme funds heat pumps and biomass boilers only. Low-income households in England may qualify for fully funded PV through the Warm Homes: Local Grant, and government-backed low-interest loans for solar are being rolled out under the Warm Homes Plan. Both technologies get 0% VAT until 31 March 2027, and only PV earns export income.
Updated 22 August 2026 with MCS installation data through the first half of 2026, Energy Saving Trust field-trial figures on real-world solar fractions, and the Ofgem July to September 2026 price cap.
The better technology and the worse investment
Solar thermal captures two and a half times more energy per square metre of roof than photovoltaic panels. But a collector costs roughly five times as much per square metre, and it makes heat, which cannot be stored in a battery, sent to a car or sold to the grid, in a decade when every new load arriving in the British home has been electrical.
Work out the cost of a kilowatt-hour of your hot water, then measure your usable roof. Above 15p and under ten square metres, fit a collector. Anywhere else, fit panels and add a diverter for £500.
METHODOLOGY: Thermal yields modelled at 450 kWh per m² per year for evacuated tube collectors on a south-facing 35-degree roof, with real-world solar fractions taken from published Energy Saving Trust field-trial findings. PV yields modelled at 850 kWh per kWp per year and 0.21 kWp per m². Values use the Ofgem July to September 2026 price cap of 26.11p per kWh for electricity and 7.33p for gas, with 90% boiler efficiency, a 12p export rate and 50% self-consumption unless stated. Installation figures are MCS certified installation counts as reported for 2024, 2025 and the first half of 2026.
DISCLAIMER: Scores are our editorial judgement. Outcomes vary considerably with roof orientation, shading, occupancy, hot water use and tariff; ask installers for an MCS calculation specific to your property before deciding. Prices, VAT treatment and scheme eligibility change; verify the current position before committing. We do not provide financial advice.
