Solar thermal and solar photovoltaic are two different things that sit on the same roof. Thermal collectors heat water directly and feed a cylinder; photovoltaic panels make electricity. Thermal is more efficient at what it does and does only that one thing, which is why it has largely lost ground in Britain to PV plus a diverter or a heat pump.
What a solar thermal system is
Collectors on the roof, either flat plate panels or evacuated tube collectors, heat a fluid that is pumped through a coil in a twin coil hot water cylinder. A controller runs the pump when the collector is hotter than the cylinder. The boiler or immersion tops up the rest. It needs a compatible cylinder, which is the part that most often makes a retrofit awkward, because a combi boiler system has no cylinder at all and one has to be found space for.
Flat plate against evacuated solar tubes
Flat plate collectors are cheaper, more robust, lower profile and less efficient per square metre. Evacuated tubes are insulated by their vacuum, work better in cold and diffuse conditions which suits a British winter, cost more, and individual tubes can be replaced when damaged. Tubes sit higher off the roof and are more visible. Neither is right in all cases, and the choice is usually made on roof area, budget and appearance.
Why most households now choose PV instead
A thermal system does one job: it heats water, well, for part of the year, and it cannot do anything else with the energy. A photovoltaic array makes electricity that can heat water through a diverter, run the house, charge a car, or be exported for payment. PV prices have fallen a long way, thermal has not fallen as far, and cylinders are less common than they were. Thermal still makes clear sense where hot water demand is high and constant, such as a guest house, a care setting or a household with a large family and an existing cylinder.
Local solar thermal installers, servicing and what goes wrong
Thermal systems have a pump, a controller, a pressurised circuit and a heat transfer fluid that degrades over time, so unlike PV they do need periodic servicing: fluid checks and replacement, pressure, pump and controller operation, and the cylinder's own maintenance. Neglected systems stagnate in summer, which degrades the fluid, and a system left unserviced for a decade is often found not working at all. Budget for a service interval and use an installer who fits them regularly rather than one who fits PV and occasionally sees one.
Questions people ask about solar thermal
What is the difference between solar heating panels and solar PV panels?
Solar thermal collectors heat water directly and feed a hot water cylinder. Photovoltaic panels make electricity, which can then do anything electricity does, including heating water through a diverter. They are different technologies that look superficially similar on a roof.
Can solar panels heat a house?
Not directly in any practical British sense. Photovoltaic panels can run a heat pump or an immersion heater, and solar thermal heats water rather than the house. Space heating from roof mounted collectors alone is not a realistic proposition here.
Is solar thermal worth it?
It can be where hot water demand is high and year round and a suitable cylinder already exists. For most households photovoltaic with a diverter is now the more flexible answer, because the same roof area does more than one job.
Does solar thermal need servicing?
Yes, unlike photovoltaic. The circuit has a pump, a controller and a heat transfer fluid that degrades, so periodic checks and fluid replacement are part of owning one.