Solar hot water, also called solar thermal, uses roof-mounted collectors to heat water directly rather than generating electricity. It is a different technology from the solar panels most of this site covers.
We should be straight with you: for most UK households it is no longer the obvious choice, and the reasons are worth understanding before you spend money on it.
How it differs from solar panels
| Solar thermal | Solar PV | |
|---|---|---|
| What it produces | Hot water | Electricity |
| What it offsets | Your water heating fuel | Anything electrical |
| Roof area | Smaller | Larger |
| Can be paid for surplus | No | Yes, via the Smart Export Guarantee |
| Works in winter | Poorly | Poorly, but the electricity is useful year round |
| Typical lifespan issues | Pump, fluid, controller | Inverter |
The key difference is flexibility. Solar thermal can only heat water. If your cylinder is already hot, the surplus is wasted. Solar PV generates electricity that can heat water, run appliances, charge a car, run a heat pump, charge a battery, or be exported for payment.
Why the picture has changed
Three things have moved against solar thermal since most content about it was written:
1. Solar PV got much cheaper The cost per unit of useful energy has fallen far more for PV than for thermal.
2. The Renewable Heat Incentive closed The domestic RHI, which paid for solar thermal, closed to new applications on 31 Mar 2022. There is no equivalent payment for solar thermal now. The Boiler Upgrade Scheme that replaced it covers heat pumps and biomass, not solar thermal.
3. Solar diverters became common A diverter sends surplus solar electricity that would otherwise be exported into your immersion heater. It is relatively cheap, it uses your existing cylinder, and it achieves much of what solar thermal does without a second roof system. See how much you will export.
Source: DHRI closure. Checked 4 Nov 2026.
Where solar thermal still makes sense
It has not become useless. It is genuinely good at what it does.
- High hot water demand, such as a large household, or premises using a lot of hot water
- Limited roof space where you want the best hot water output from a small area. Solar thermal collects more usable heat per square metre than PV generates electricity
- You already have it and it is working. A functioning system is worth maintaining rather than replacing
- Off grid properties where electricity is scarce and hot water is not optional
If you have an existing system
Solar thermal systems need more attention than PV:
- The pump and controller wear out
- The heat transfer fluid degrades and needs replacing periodically
- Pressure needs checking
- The expansion vessel can fail
A system that has been ignored for a decade may not be performing. If yours has stopped working, get it assessed before assuming it needs replacing. Often it is the pump or the fluid.
What most households should consider instead
Solar PV with a diverter, which heats your water from surplus generation and does everything else too. See what solar panels cost.
A heat pump, if you are rethinking your heating and hot water together, which also carries a substantial grant. See heat pumps.
Both, which is where the strongest case now sits. See solar and a heat pump together.