Solar water heating offers a way to use the sun’s power to warm a liquid directly, providing a green alternative to electric heating systems. This technology uses solar collectors to absorb heat from the sun and transfer it to the water used in a home. The process involves harvesting free energy to handle daily tasks like bathing and washing clothes, working alongside conventional water-heating systems.
Modern building techniques have made these installations more robust and accessible to the average household.
How the panels work
Solar-thermal panels, also known as collectors, capture sunlight and convert it into heat. This energy warms a fluid—typically a water-antifreeze mixture—circulating through the panels. The heated fluid then transfers the energy to the home’s water supply via a heat exchanger in a hot-water cylinder or thermal store.
There are two main types of collectors: flat-plate and evacuated-tube. The latter are generally more efficient because they retain heat better thanks to their vacuum-sealed design. To function effectively, the collectors must be installed where they receive sunlight for most of the day, usually on a south-facing roof. However, roof space or orientation can be an issue, so ground or wall-mounted systems are also options.
For these systems to work, the roof angle should ideally be between 20 and 50 degrees. It is also important to ensure there is enough structural support, as evacuated-tube collectors can be heavier than flat-plate systems. The installation requires careful consideration of these physical constraints to ensure the equipment remains secure and operational.
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Compatibility with existing systems
Jon Bonnar, of Cotswold Energy, explains that the system works all year round, whether the sun is shining or it is mid-winter. However, you are likely still to need to heat the water further with a boiler or immersion heater, especially during lower-daylight months. During the summer, the system can provide 90% of hot water, dropping to around 25% throughout the winter due to low light levels.
William Hobbs notes that a combination of solar thermal with conventional gas, LPG and oil boiler systems and heat-pump systems works well because these use hot-water cylinders. Making solar thermal compatible with a combi boiler is a bit more complex. Typically, you’ll need to install a thermal store or hot-water-cylinder and a diverter to allow you to bypass the boiler when you want to use water from it.
This setup presents two main issues: you need to find space to fit the cylinder, and you must find an MSC-certified heating engineer to set it up. The installation is straightforward in theory, but the practical limitations of many homes can make it difficult to implement. The space required for a thermal store is often a significant barrier for homeowners with smaller properties.
Costs, savings, and alternatives
A well-installed solar-thermal system used properly can drastically cut hot-water bills by up to 70%. The Energy Saving Trust estimates savings in the region of £120 to £225 a year for an average 4m2 collector system. The cost of installing a solar-thermal system in the UK ranges from £3,000 to £8,000, with an average cost of around £4,500. This includes the panels, hot-water cylinder, control panel, and installation.
DIY kits are available at lower costs, around £1,750, but professional installation is recommended to ensure system efficiency. Maintenance costs are relatively low, making solar thermal an economical long-term investment. However, the initial investment will take a long time to pay back. In strictly financial terms, you may not get your money back over the lifespan of the equipment, which can be up to 10 years depending on where they are installed and how they are used.
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If you already have a conventional solar-power set-up, there is another option for heating water. An immersion power diverter, sometimes called an immersion optimiser, is a gadget costing £700-£1,000 that diverts excess energy from standard solar panels towards your immersion heater to heat water. If you have a conventional boiler and hot-water cylinder system, you could install this to maximize your solar-panel usage.
The resultant reduction in energy bills means that you’ll have effectively paid off the cost of installing an immersion power diverter within two years. However, the Smart Export Guarantee allows energy suppliers to decide what tariffs to offer their customers. This means it may not be financially beneficial if you’re on this scheme, so it is worth checking the rate you’re getting for your excess energy. Also, you won’t be able to invest in one of these if you have a combi boiler.
Solar-thermal panels significantly reduce greenhouse gas emissions. A typical system can prevent up to 930kg of CO2 emissions annually. Over a decade, this can mean a reduction of 3,100kg to 9,300kg of CO2, depending on your existing heating system. This makes solar-thermal panels an excellent choice for environmentally conscious homeowners looking to reduce their carbon footprint.
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