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.
How it works
Solar thermal uses collectors on the roof, either flat panels or evacuated glass tubes, through which a fluid circulates. The sun heats that fluid, which is pumped down to a coil inside your hot-water cylinder, transferring its warmth to the water you actually use. It needs a compatible cylinder, usually one with two coils so the boiler or immersion can top up the heat when the sun cannot, and it works alongside your existing heating rather than replacing it.
What it delivers
On a sunny day in the warmer half of the year, solar thermal can provide most or all of a household's hot water, which is a meaningful chunk of the energy bill given that heating water is often the second largest use in a home. In winter it delivers far less, because there is less sun and the incoming water is colder, so your boiler does most of the work then. Realistically it covers a good share of your annual hot water, concentrated in spring through autumn, rather than the whole year.
Solar thermal versus solar PV
This is the live question, because roof space and budget are limited. Solar thermal is very efficient at its one task of heating water, more so per square metre than PV is at the same job. But PV is far more flexible: it makes electricity you can use for anything, including heating water through an immersion via a simple diverter, as well as running the house and charging a car. Because of that flexibility, and falling panel prices, many households now fit PV instead and use some of its output for hot water, getting water heating as one benefit among several rather than a single-purpose system.
Is it worth it?
Solar thermal still makes sense in specific situations: where hot-water demand is high, where you want the most efficient possible water heating from limited roof space, or where you are replacing the cylinder anyway and the system can be fitted neatly. For many others, PV with a hot-water diverter is the more versatile use of the same roof and money. As with any larger investment, do the cheap efficiency basics first, get a proper assessment for your specific roof and demand, and treat the payback as long and steady rather than quick.
The bottom line
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 and much else, which is why PV has become the more popular choice. If hot water is your priority and the fit is right, solar thermal still earns its keep; for most, PV is the more adaptable option.