How solar thermal works
Solar thermal works through water in panels on your roof, which absorb the sun’s heat. The hot water is then pumped to a hot water tank or cylinder to be stored until needed. It does not require bright sunlight in order to function; while that would be the ideal, it can still heat water even on cloudy days.
You can add solar thermal panels to most existing hot-water systems.
However, you’ll usually need to add an additional hot-water cylinder for the solar-heated water alongside the cylinder connected to your boiler or heat pump.
Or, to save space, you can change your existing cylinder for one with two heating coils.
Solar thermal can be combined with a conventional boiler or immersion heater to make the water hot enough to use in radiators, or to provide hot water when solar energy is unavailable especially during winter.
The downside is that it may not work well with a combi (water on demand) boiler. It depends on whether your make of boiler can accept pre-heated water.
Around half of combi boilers won’t do that. If you’ve got one of them you’ll either need to change it to a conventional boiler or use another technology, like heat pumps or PV instead of solar thermal.
What does it cost?
The cost of installing a typical solar water heating system is £4,000-£5,000. EST found that typical savings from a well-installed and properly used system are moderate – £60 a year when replacing gas heating, and £70 a year when replacing electric immersion heating.
But it may be eligible for the revised Green Homes Grant if you are on a low income. And, once it has been installed, you can receive payments through the Renewable Heat Incentive (RHI) per kWh you generate.
For applications submitted between 1 July 2020 and 30 September 2020, owners of a solar thermal heating system received the highest payments.
|Type of renewable heating system
|Air Source Heat Pump
|Ground Source Heat Pump
However, according to CAT:
“Bear in mind that the [green homes] grant value will be deducted from any RHI payments .... If you need to fund substantial insulation improvements, it would therefore be more cost-effective to use the grant for those.”
Homes with a hot water cylinder and high demand for domestic hot water, such as a family or house share, are the best suited to a solar thermal system. For a one-person household, with low hot water use, it won’t be as cost-effective.
The types, and installation details
There are two types of solar thermal panel, which are known as collectors:
- Flat plate – resemble solar PV panels and sit flat on the roof
- Evacuated tubes – a bank of glass tubes mounted on the roof tiles.
Evacuated tubes are more expensive than flat plate. They are often regarded as more efficient, but the Centre for Alternative Technology (CAT) says that with UK weather, there isn’t a big difference.
The Energy Savings Trust (EST) found that the main impact on a system’s performance are how well insulated the home’s hot water tank and pipes are.
One downside of solar thermal is that the panels have to be connected to the plumbing, so installation is not straightforward. Solar Guide thus says that a solar thermal system is best installed as part of a renovation or during the construction of a property. But, once installed, maintenance costs for solar thermal systems are generally very low and they will last for 20-25 years.
It is recommended that you find a professional installer rather than trying to do it yourself. The government mandated Microgeneration Certification Scheme is a quality standard covering installers of certain low-carbon technologies, in particular heat pumps and solar PV. Whilst not an absolute guarantee of quality, MCS offers a mechanism for best practice, advice and rectification.
Many incentive schemes like the Renewable Heat Incentive (RHI) require MCS qualified installers, so look out for the standard. For that reason, we have only covered MCS-certified brands in this guide.
Solar thermal vs heat pumps
Heat pumps can do space heating and hot water, so they are able to be more of a complete solution. But you can combine them, and there may sometimes be advantages to doing so. Air-to-air heat pumps only do space heating, so in this instance you could get solar thermal to help with your hot water.
Solar thermal vs solar PV
The market for solar thermal has declined since solar PV became cheaper and more popular. Many solar thermal companies stopped trading. Installing solar thermal systems was part of the short-lived Green Homes Grant, and solar PV isn’t, so check locally if there are any new or expanded green grant schemes which might be applicable.
Solar water heating is more efficient than using solar PV panels to heat water.
This means solar thermal panels will take up much less roof space than PV panels would for the same energy output, because you’ll need fewer of them.
In addition, there is a financial saving – solar thermal panels are generally cheaper than PV ones.
But because solar thermal needs a hot water cylinder and works better with a conventional or ‘system’ boiler, if you have a combi boiler and no hot water cylinder, solar PV may be better.
Of course, if you have a big enough south-facing roof, or you can mount panels at ground level where they’ll have a clear outlook, you can have both systems, either as two separate systems or Solar PV-T – which combines both.
But Solar PV-T systems aren’t yet as popular as solar PV or solar thermal systems and there are only a handful of manufacturers, none of which have been covered in the guides in this magazine. These hybrid systems are also not eligible for the Renewable Heat Incentive (RHI).
Make the most of your solar water system
Which? has the following advice:
- Use as much hot water as possible from the solar-heated supply, rather than heating it up in an appliance using electricity. For example, fit a mixer shower (rather than electric) and check whether any ‘hot-fill’ appliances (which could include your washing machine) can take water from your solar system.
- Use more hot water in the evenings – in showers, baths and washing-up. This is when water will be hottest, as it’s had most time to heat up.
- Insulate your pipes and water tank to make your system more efficient.
- Follow your installer’s advice on how to set the hot water controls to get the most from your system.
Other uses for solar thermal
Solar water heating can be used to provide warm water to a site which has no facilities at all: allotments, garden sheds, and campsites. The simplest is a ‘solar shower’ black pouch with a tap at the bottom, which costs less than £10 and provides ample hot water for camping.
Solar water heating is also popular for heating swimming and paddling pools.