When people say solar these days they usually mean the electricity-generating panels covered in the solar basics guide. But there is an older, more single-minded technology that is easy to forget: solar water heating, or solar thermal, which uses the sun to heat your hot water directly rather than to make electricity. It does one job, and for that job it does it well. The live question is whether it still earns its place on a roof that could carry solar panels instead. This guide works through what it delivers, what it saves and when each option makes more sense.
The short answer. Solar thermal uses roof collectors to heat a fluid that warms the water in your hot-water cylinder. On a sunny day from spring to autumn it can supply most or all of a household's hot water, covering perhaps 50 to 60 percent of annual demand, with the boiler topping up in winter. It is very efficient at that one task. But solar PV makes flexible electricity that can heat water through a diverter and run everything else too, which is why many households now fit PV instead. Solar thermal wins where hot-water demand is high or you are replacing the cylinder anyway.
How solar thermal works
Solar thermal uses collectors on the roof, either flat-plate panels or evacuated glass tubes, through which a fluid circulates. The sun heats that fluid directly. A small pump sends it down to a coil inside your hot-water cylinder, where it gives up its warmth to the water you actually use, then returns to the roof to be heated again.
The two collector types differ a little. Flat-plate panels are cheaper and robust. Evacuated tubes are more efficient in cold and dull conditions, because the vacuum around each tube cuts heat loss, but they cost more. Either way the system needs a compatible cylinder, usually a twin-coil one, so that your boiler or immersion heater can heat the same water from a second coil when the sun cannot. It works alongside your existing heating rather than replacing it.
It is worth being clear that this is heat, not electricity. The system makes hot water and only hot water. That focus is both its strength and its limitation, as the rest of this guide explains.
What it actually delivers across the year
On a sunny day in the warmer half of the year, solar thermal can provide most or all of a household's hot water. That is a meaningful chunk of the energy bill, because heating water is often the second largest use of energy in a home after space heating.
The catch is the seasonal swing. In winter it delivers far less: there is less sun, the days are short, and the incoming mains water is colder, so there is more work to do from a smaller solar input. Your boiler does most of the heating then.
| Season | Share of hot water from solar | What tops it up |
|---|---|---|
| Summer | 80 to 100% | Little or nothing |
| Spring and autumn | 40 to 70% | Boiler or immersion |
| Winter | 10 to 25% | Mostly boiler or immersion |
| Whole year | ~50 to 60% | Boiler or immersion |
Indicative shares for a typical UK household and roof. Actual figures depend on roof orientation, collector size, cylinder size and your hot-water habits.
So realistically it covers a good share of your annual hot water, concentrated from spring through autumn, rather than the whole year. That is genuinely useful, but it is not the off-the-grid hot water some imagine. Pairing it with a well-lagged cylinder so the heat it makes is not wasted, as in cylinder jacket and pipe lagging, gets the most from every sunny hour.
Solar thermal versus solar PV
This is the question that decides most installs, because roof space and budget are both limited and the two technologies compete for the same pitched roof.
Solar thermal is very efficient at its one task. Per square metre of roof, it captures more of the sun's energy as useful hot water than solar PV does when PV is used to heat water. If heating water is all you care about, thermal makes the most of a small roof.
But solar PV is far more flexible. It makes electricity you can use for anything: running the house, charging a car, and yes, heating water through a simple device called a diverter that sends surplus generation to the immersion heater instead of exporting it. So PV gives you hot water as one benefit among several, rather than a single-purpose system. With panel prices having fallen a long way, many households now fit PV and use a diverter for hot water, getting water heating thrown in alongside everything else the electricity can do.
| Solar thermal | Solar PV with diverter | |
|---|---|---|
| What it makes | Hot water only | Electricity for anything |
| Efficiency at heating water | Higher per square metre | Lower per square metre |
| Flexibility | None beyond hot water | House, car, water, export |
| Cost trend | Steady | Falling fast |
The full case for panels is in solar panels: the basics and is solar worth it, and if you go down the PV route, solar battery storage is the next question.
When solar thermal still makes sense
Despite PV's flexibility, solar thermal still earns its keep in specific situations. It is not a dead technology, just a more specialised one.
- High hot-water demand. A large household, or one with high usage, gets more out of a system dedicated to water, because there is always a tank to fill.
- Limited roof, water the priority. If you have only a small patch of suitable roof and hot water is what you most want, thermal extracts more useful water heat per square metre than PV would.
- Replacing the cylinder anyway. If your hot-water cylinder is due for renewal, fitting a twin-coil solar cylinder at the same time folds much of the disruption and some of the cost into work you were doing regardless.
- No interest in the wider electric uses. If you have no EV, no plan for one, and modest electricity use, the flexibility of PV is worth less to you, which tilts the balance back towards thermal.
For many other households, PV with a hot-water diverter is the more versatile use of the same roof and money.
Do the cheap things first
As with any larger investment, the order matters. Before spending four figures on any solar system, make sure you are not heating water you then waste.
A well-lagged cylinder and lagged pipes keep the heat you make, whether it comes from the sun or the boiler, covered in cylinder jacket and pipe lagging. Cutting how much hot water you use in the first place, with a low-flow showerhead and the habits in hot water savings, shrinks the demand any solar system has to meet, which makes a smaller, cheaper system go further. If you heat water with an immersion, knowing the immersion heater cost tells you what the solar input is actually displacing.
Done in that order, the cheap fabric and habit fixes cut the bill first, and whatever solar system you then fit has less work to do and a better payback.
The verdict
Solar water heating is a proven, efficient way to cover much of your hot water from spring to autumn, working alongside your boiler for the rest. Its limitation is that it does only that one thing, while solar PV makes flexible electricity that can heat water through a diverter and power much else besides, which is why PV has become the more popular choice.
If hot water is your priority, demand is high, and the fit is right, especially if you are replacing the cylinder anyway, solar thermal still earns its keep. For most others, PV with a hot-water diverter is the more adaptable use of the same roof and budget. Either way, treat the payback as long and steady rather than quick, do the cheap efficiency basics first, and get a proper assessment for your specific roof and demand before committing. The wider solar picture is in solar panels: the basics and is solar worth it.