Solar water heating with solar thermal panels

A solar water heating system uses solar thermal panels on your roof to heat water to use around your home.

Fitting this type of water heating system isn’t cheap so, before you invest, you need to find out whether solar thermal panels are right for your home and your needs.

Our expert advice will tell you how the system works, what you need to consider, and help make sure you get a good price from a reputable installer.

If you want to generate your own electricity, head to our guide to solar PV panels instead.

How do solar thermal panels work?

Solar thermal panels use heat from the sun to warm fluid passing through them, as the diagram below shows.

This is then used to heat your water, which is stored in a hot water cylinder. An immersion heater/unvented hot water cylinder might be needed as a back-up heater or to get the water to the temperature you want.

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Types of solar thermal panels

There are two main types of solar water heating panels – flat plate and evacuated tubes (referring to the way the water interacts with the panel). 

Evacuated tubes look like a bank of glass tubes fitted to your roof. Flat plate systems can either be fitted onto the roof or integrated into it.

Evacuated tube systems are more efficient than flat-plate versions, so are often smaller but still generate the same amount of hot water.

There are also drainback systems, which drain water from inside the solar panel when the pump is switched off. This prevents water from freezing or boiling inside the solar panel.

How much do solar thermal panels cost?

Solar thermal panels typically cost between £3,000 and £5,000 to install.

For comparison, a conventional gas combi boiler costs between £1,350 and £6,300 to install. 

This will vary depending on how much work is required, the parts needed, availability, where you live and who you employ to do the job.

Find out more about how much a new boiler costs.

Once installed, additional costs are minimal. Most systems use a small amount of electricity to run the pump but, in most cases, the pumping costs cancel out only about 8% of savings, and newer technology can typically reduce this to 3% or even zero.

You’ll need to bear in mind the cost of maintenance, although it's generally very low for solar water heating systems. For example, a replacement pump costs around £90 to replace, while new anti-freeze costs around £100, and they only need servicing every three to seven years.

Most systems come with a five or 10-year warranty.

Can I save money with solar water heating?

Savings with a solar water heating system are fairly moderate. How much you can save depends on the type of water heating system you are replacing. Bear in mind that the system should be able to provide most of your hot water in summer, but much less in winter.

As a general guide, you could save around £145 to £275 per year on your water bills with solar thermal panels based on current fuel prices under the Energy Price Guarantee, which applies until the end of June 2023. 

You’ll save the most if you are currently using electricity to heat your water – around £275 per year. If you are replacing gas central heating, savings are likely to be closer to £160 per year.

Annual bill savings with solar water heating

• Oil – £145
• Coal – £155
• Gas – £160
• LPG – £190
• Electricity – £275

_{Data from Energy Saving Trust.}

We recommend that you first make sure your home is as energy efficient as possible, then think about which types of renewable energy technology, including solar panels, might suit you.  

And remember: the more energy efficient your house is, the less heat you'll lose and, therefore, the less heat you'll need to generate in the first place. This means that you should need a smaller heating system, which will be cheaper to buy and to run.

Are solar thermal panels right for your home?

For maximum efficiency, you need to put your solar thermal panels on a south-facing roof at a 30-degree angle to the horizontal (up to 65 degrees will still work in the UK). 

Panels should be kept away from shadows (trees, buildings, chimneys, etc).

Solar panels can be heavy, so your roof must be strong enough to take their weight, especially if the panel is to be installed on top of existing heavy tiles.

Not all boilers are compatible with solar water heating – for example, if you have a combi boiler and you don’t have a hot water tank. So check what extra equipment you’ll need – and how much it will cost – if you are considering solar water heating.

Some panels require regular checks of the unit and connections, or a wipe of the panel glass with mild detergent. Your installer should leave written details of any maintenance checks you should carry out from time to time. Bear in mind how difficult this might be when panels are up on your roof. 

Pros and cons of solar water heating

Pros

• Solar water heating can provide you with about 90% of your hot water needs in summer and 25% in winter.
• You could save between £145 and £275 per year on your fuel bills.
• Systems need little maintenance and the on-going costs are very low.
• Most solar water heating systems come with a five to 10-year warranty.
• Lowers your carbon footprint.

Cons

• You'll still need a boiler or immersion heater to make the water hotter, or provide hot water when solar energy isn't available.
• Not all boilers are compatible with solar water heating.
• Solar thermal panels can cost more to install than conventional electric and gas heating systems.

How to choose a solar water heating system

When choosing a solar water heating system, you'll need to consider four major factors:

• your average hot water use;
• the area of south-facing roof available;
• your existing water heating system;
• your budget.  

You'll need roughly one to two square metres of panel area per person in the household. Each metre of panel area will need between 30 and 60 litres of water-tank volume.

If you use a less efficient panel (such as flat-plate solar thermal panels), you'll need to cover a larger area than if you use a more efficient one, such as evacuated tubes.  

You'll also need to select system components – such as a hot water cylinder, controls and pipe work – and choose the location for your solar panels, considering shade, pipe runs, roof pitch and future access.

Finding a good solar panel company — things to look out for

You'll need to hire a professional to install your solar water heating system. There are plenty of solar panel installers out there, so we recommend that you always collect a range of quotes to compare.  

Watch out for dodgy sales tactics, salespeople putting you under pressure to buy on the spot, and exaggerated financial savings. We recommend you do your own research before inviting a company into your home.

You should only use installers and products that are certified by the Microgeneration Certification Scheme (MCS), then compare the estimates of costs and savings you're given by salespeople against other sources of advice.

You can search for a certified installer on the Microgeneration Certification Scheme website.

Which? Trusted Traders will help you find a trustworthy local solar panel installer.

What you need to know about solar thermal panel installation

A solar water heating system involves pipe work, a thermostat and a hot water cylinder.  

Some also have a drainback system to drain water from inside the solar panel when the pump is switched off.

You can add solar thermal panels to most existing hot water systems. However, you'll usually need to add an additional cylinder for pre-heated water or change your existing cylinder for one with a twin coil.

It's difficult to use a solar water heating system with a combi boiler. This is because combi boilers heat water directly from the mains water supply and don't have a tank; solar water heating systems supply warm, low-pressure water. Some new combi boilers do accept pre-heated water, so check with the manufacturer.  

To find out which boilers are the most reliable, take a look at our guide to the most reliable gas and oil boiler brands.

How to get the most from your solar thermal panels

Before you install solar water heating, try to reduce the amount of hot water you use, to see if you can reduce costs without it.

Or make the most from your solar hot water system by following these tips:

• Use as much hot water as possible from the solar-heated supply, rather than heating it up with 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 if it's relatively old) can take water from your solar system.
• Consider changing your behaviour to 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.
• Ensure your back-up system is set up to switch on at the right time. Before you installed solar water heating, it was probably set to give you a full tank of hot water in the morning. But if you don’t change it, your panels will have nothing to heat during the day.
• Insulate your pipes and water tank to make your system more efficient.
• Make sure your water tank is heated to more than 60% at least once a week to avoid bacteria building up.
• Follow your installer’s advice on how to set the hot water controls to get the most from your system.

You should also make sure that you carry out any maintenance checks from time to time to make sure the system is working efficiently. Your installer should leave written details of these.

The most important thing to check for is leaks. If your system leaks antifreeze, you will probably be able to smell it – contact your installer.

During the lifespan of your panels, the anti-freeze may need replacing or topping up. This costs around £100.

Your installer may specify that your system needs checking by an accredited professional every few years. They will also check the pump. These last for around 10 years and cost around £90 to replace.

Do you need planning permission for solar thermal panels?

You don't need planning permission for most domestic solar water heating systems in the UK, as long as they aren’t too big.

But exceptions apply for listed buildings, buildings in conservation areas and World Heritage Sites.  

If this applies to you, contact your council to see whether you need to apply for planning permission for your solar panels.