Solar PV vs solar thermal

Both technologies put glass on your roof and harvest sunshine, but that's where the similarity ends. Solar PV (photovoltaic) generates electricity. Solar thermal generates hot water directly. The case for one over the other has shifted dramatically in the last five years, and this article walks through where it lands in 2026.

The short version: PV is now decisively the better choice for the majority of UK homes. Solar thermal isn't a bad technology — it works, it produces hot water reliably, and it lasts for decades — but the relative economics moved against it as PV prices crashed and electricity tariffs evolved. Most installers won't even quote solar thermal anymore unless you ask specifically.

What each technology actually does

Solar PV uses semiconductor cells to convert sunlight directly into DC electricity, which an inverter then turns into AC for your house and grid. A typical UK 4kW PV system generates around 3,800–4,200 kWh per year, depending on orientation and shading.

Solar thermal uses either flat-plate collectors or evacuated tubes to heat a fluid (glycol mix), which then heats a hot water cylinder via a heat exchanger. A typical 4 m² flat-plate system delivers 1,500–2,200 kWh/year of hot water heat in the UK, covering roughly 50–60% of an average household's annual hot water demand.

The fundamental difference: PV produces a flexible commodity (electricity) that can do any job, including heating water if you want. Solar thermal produces a single product (hot water) with no ability to redirect surplus. That single difference drives most of the economic verdict.

Install cost comparison

The diverter option is the one most people miss. A device like the myenergi Eddi or Solic 200 sits between your PV inverter and your hot water immersion heater. When your panels are generating more than your house is using, instead of exporting it to the grid for 5p/kWh, the diverter dumps it into your hot water cylinder. You get free hot water from surplus solar without needing a separate solar thermal system.

The diverter only works if you have a hot water cylinder with an immersion heater (most system boilers, all unvented cylinders). Combi boilers don't have one, which has historically been an obstacle. The simple workaround: add a small (90L) hot water cylinder with an electric immersion. The cylinder costs £400–£700 plus install; combined with a £350 diverter that's still significantly cheaper than a solar thermal system and you get all the PV-flexibility advantages.

Payback in 2026

Solar PV payback has improved with the increase in electricity unit rates, even after the demise of the Feed-in Tariff. A 4kW PV system today saves roughly £700–£1,000/year through self-use plus Smart Export Guarantee (SEG) payments at 8–15p/kWh. That's a 7–11 year payback on a £6,000 system.

Solar thermal saves roughly £200–£350/year for a typical household — assuming gas is being displaced at roughly 7p/kWh delivered heat. On a £5,000 system, that's a 14–25 year payback. The headline numbers haven't moved much in the last decade because gas is still relatively cheap.

The economics aren't close. PV plus a £400 diverter handles your hot water through the sunny months and gives you electricity all year. Solar thermal does only one job and does it for fewer months.

The 20-year case for solar thermal got worse, not better, between 2015 and 2025. PV panels dropped from £1.50/W installed to £0.80/W. Solar thermal collectors dropped from about £450/m² to £400/m² over the same period — barely any movement. Battery prices halved. Inverters got smarter and cheaper. Solar thermal didn't improve materially. It's now an aging technology in a market that has moved on around it.

Why PV plus heat pump is the modern answer

If you're considering a heat pump (and most readers of this site are), the calculation gets even more decisive. A heat pump heats your hot water at SCOP 2.5–3.0 in summer (lower than space heating because cylinder temperatures are higher). Driving that heat pump with surplus PV electricity means your hot water effectively costs nothing for 4–5 months of the year, plus your space heating gets cheap PV electricity on sunny shoulder-season days.

Solar thermal can't do any of that — it produces hot water and only hot water, and it can't export surplus when your cylinder is full. Once the tank reaches setpoint, the system stagnates and the panels sit there doing nothing.

An additional advantage of the PV-and-heat-pump combination: future-proofing. As the UK electrifies further (EVs, induction cooking, heat pumps), having more electricity-generation capacity on your roof becomes more valuable, not less. Solar thermal can't be redeployed to charge an EV or feed an induction hob. Roof real estate spent on thermal is roof real estate that can't pull double duty.

Where solar thermal still wins

We'd rather be honest about edge cases than pretend they don't exist. Solar thermal can still be the right call in:

• Very high hot water demand: Households of 6+ people with constant hot water draw can consume the thermal output instead of stagnating.
• Off-grid properties: Where electricity import is constrained or expensive, thermal can sidestep the inverter and battery cost path.
• Small roofs unsuitable for meaningful PV: A 4 m² thermal collector can fit where a 4kW PV array can't, but you'd genuinely need to have ruled out PV first.
• Existing thermal store with spare coil: Adding thermal to a pre-existing thermal store cylinder with an unused coil is cheaper than retrofitting PV.
• Listed buildings with PV planning issues: Some heritage areas accept thermal collectors more readily than PV arrays.

For the average UK household — three or four people, a standard combi or system boiler, a typical roof — these conditions don't apply, and PV with a diverter or PV with a future heat pump is the better economic and decarbonisation play.

One nuanced case worth flagging: if you've already got solar thermal that's working well and you're considering whether to extend it or add PV alongside, the answer is almost always to add PV alongside. Don't rip out the working system unless it's failing. The thermal panels keep delivering free hot water for as long as they survive (often 20+ years) and the new PV starts paying for itself on day one of operation.

Practical issues most articles skip

Stagnation. Solar thermal systems can overheat in summer when your cylinder is full and you're on holiday. The glycol fluid degrades faster, and badly designed systems vent steam. Modern drain-back designs handle this, but not all installers fit them.

Antifreeze maintenance. Glycol needs replacing every 5–8 years at a typical cost of £200–£350. PV has no equivalent fluid maintenance.

Roof penetrations. Both systems need penetrations, but thermal carries hot fluid at high pressure and the consequences of a leak are worse. Insurers and surveyors increasingly raise eyebrows at older thermal installations.

System lifespan. PV panels are warrantied 25 years, inverters 10–12 (replace once during system life). Solar thermal collectors last 20–25 years, but pumps, controllers, and the glycol loop have shorter service lives.

Insurance and warranties. PV is well-understood by home insurers and rarely affects buildings premiums. Some insurers raise older solar thermal installations as a query because the high-pressure glycol circuit is seen as a leak risk in roof-voids. Always disclose either system to your insurer.

Roof orientation. Both technologies work best south-facing, but PV is much more tolerant of east-west splits — modern microinverters and optimisers minimise the penalty for partial shading. Solar thermal performance falls off more sharply away from south because the temperature differential drives the system harder.

FAQ

Can I have both?

Technically yes, practically rarely worth it. The roof space, the budget, and the plumbing complexity usually mean you should pick one. If you have abundant roof space and budget, a larger PV array beats splitting roof area between two technologies.

Will solar thermal work with my combi boiler?

Combi boilers heat water on demand, so they don't have a cylinder for solar thermal to charge. You'd need to add a thermal store or twin-coil cylinder, which adds £1,500–£2,500 to the install and complicates plumbing. This is the main reason thermal has lost ground — most UK homes now have combis.

What about pool heating?

Solar thermal works well for swimming pools because pools tolerate lower temperatures and have huge thermal mass. Different system, different economics — outside the scope of a domestic hot water comparison.

Does PV need a battery?

No, but a battery improves self-consumption from ~30% to 60–70%, which is meaningful when import rates are 25–30p and export is 8–15p. Whether it pays back in your specific case depends on your usage profile — see our battery sizing tool.

What's the carbon comparison?

Both technologies displace carbon. PV at typical UK generation displaces around 0.7–0.8 tonnes CO₂/year. Solar thermal at 1,800 kWh/year displaces 0.35 tonnes CO₂/year if it replaces gas. PV wins on per-£ carbon abatement by roughly 2:1.

What about new flat-plate technology?

There have been incremental improvements in flat-plate efficiency, but no breakthrough. The fundamental limitation is that thermal output is wasted once the cylinder is full, and that ceiling hasn't moved.

What this means for your decision

For most UK homeowners in 2026, the answer is solar PV with a hot water diverter, optionally paired with a heat pump down the line. Solar thermal made sense in the 2000s when electricity was expensive, gas was cheap, and PV was £15,000+ for a small system. Those conditions no longer apply. PV is cheaper, more flexible, and pairs cleanly with the heat pumps and EVs you'll likely add over the next decade.

Use the solar payback calculator on EcoSavingHub to see how PV stacks up for your specific roof, postcode, and electricity tariff.