Guide to Solar Panels and Water Heating

Solar panels are known for generating electricity to power homes and appliances. In the UK, they aren’t particularly associated with heating water. For most homes and businesses, water heating is a job for gas. 

However, solar energy can indeed be used to heat water. In theory, this lowers your carbon emissions and saves on your energy bills. But does it work in reality? Is solar water heating an effective solution? 

This guide takes a look at the different options available for solar water heating, alongside the pros and cons of each.

How does solar water heating work?

Solar water heating works by using sunlight to heat water. Not to be confused with standard photovoltaic solar panels, solar water heating is also known as solar thermal. 

There are two main ways to use solar power for heating water:

1. Solar thermal panels

Solar thermal systems use devices known as solar collectors. These are either:

• Flat-plate panels – These contain a series of pipes that are filled with a water-antifreeze mix. The liquid absorbs heat from the sun and then flows into a heat exchanger within a water storage tank. When the fluid in the exchanger is warmer than the water in the tank, heat exchange occurs. Flat-plate panels are more common and cost-effective.
• Evacuated tubes – Glass tubes with a vacuum inside. They have a heat exchanger at the top of the tubes where the heat is transferred from the solar energy. Evacuated tubes are more expensive but are more efficient. 

Both of these methods offer indirect heating of water from solar energy. Direct solar water heating would be where water that’s been heated within a solar panel flows directly into a tank. However, these systems aren’t common as they are at risk of overheating and freezing. 

2. Solar PV connected to electric water heating

Standard photovoltaic solar panels can be used to power electric water heating systems like immersion heaters or electric boilers. Essentially, an immersion heater can be connected to a PV system with some form of smart diverter, which redirects excess solar generated electricity to heat water.

This approach is flexible and it does make use of surplus electricity, but it does mean this excess electricity will be diverted away from a battery storage system or powering other appliances. 

However, it’s not the most efficient way to heat water. When it comes to solar panels for heating water, your main option should be solar thermal panels. 

How much does solar water heating cost?

Installing a solar thermal system usually costs between £3,000 and £6,000. 

This depends on the size and quality of the collectors, whether you go for flat-plate panels or evacuated tubes and if you need to install a water storage tank. 

As always with quotes, make sure you get a few from different installers so you can compare and contrast. You’ll also want to make sure the installers are registered with the Microgeneration Certification Scheme (MCS).

Pros and cons of solar water heating

Pros

• Energy savings – A solar water heating system can save households an estimated £120-£225 a year. This will reduce your energy bills over the system’s lifetime.
• Reduces carbon emissions – By reducing dependency on fossil fuels and shifting to a renewable energy source, solar water heating systems significantly lower carbon emissions.
• Minimal roof space required – Only 2-5m² of roof space is needed for most systems, which is around one panel per person in the home. 
• No planning permission – Solar water heating systems are generally considered to be permitted developments in the UK, meaning no planning permission is required for installation in most cases.
• Works well in the summer – Solar thermal systems can provide around 90% of your hot water needs in summer (but only 25% in the winter).
• Low maintenance –  Solar thermal systems only require periodic checks to make sure the system is running efficiently.
• Warranty – Most solar thermal systems come with warranties of 5-10 years.

Cons

• High initial costs – Installation costs usually range between £3,000 and £6,000 for a standard 4m² system. Given the estimated annual savings, it can take a while to reach a full return on investment – often longer than the warranty period.
• You’ll likely need a hot water cylinder – Solar water heating systems either require a hot water cylinder to store heated water or a combi boiler that accepts pre-heated water. As most homes don’t have a hot water tank, it’s likely you’ll have to buy one. 
• Partial year-round coverage – A solar water heating system won’t supply 100% of the hot water needed throughout the year. Although it provides substantial heating in the summer, in winter months you’ll likely need a boiler or immersion heater to fully meet hot water demands.
• Combi boiler compatibility – Most combi boilers don’t come with a hot water cylinder, and many aren’t designed to accept pre-heated water. This means that households with combi boilers may need to install a separate hot water cylinder to use solar thermal effectively, increasing the overall cost.
• Extended payback period – Although the savings are beneficial, the payback period for a solar water heating system can be lengthy due to the relatively moderate annual savings. It may take well beyond the warranty period to achieve full payback in some cases.

Can solar panels work with heat pumps for water heating?

Similar to solar thermal panels, heat pumps move heat via a heat exchanger. Unlike solar panels that generate electricity, heat pumps need a power source to run their mechanism. That said, heat pumps are super efficient and can generate 3-4 units of electricity for every 1 unit they consume. 

Heat pumps can work as air-to-air systems where they act as a type of air conditioning (for both hot and cold) or you can get an air-to-water heat pump. With air-to-water heat pumps, heat that’s stored in outside air can be extracted, compressed and used to heat water in a storage tank via a heat exchanger. 

For homes that want to integrate both solar thermal and a heat pump, it’s possible for an air-to-water heat pump and a solar thermal panel to feed into the same hot water tank if the tank is designed for dual inputs. Known as a dual-coil cylinder, this type of hot water tank has two separate heat exchanger coils – one connected to the solar thermal panel and the other to the heat pump.

These systems can be configured as smart systems to prioritise solar thermal input when sunlight is available. Temperature sensors monitor the tank’s temperature and switch between heat sources based on availability and demand.

Another option for homes with solar PV panels is to use the generated electricity to power the air-to-water heat pump, allowing it to cover hot water needs as necessary. This setup maximises the efficiency of solar energy, using PV power to run the heat pump when solar thermal energy is unavailable.

Is solar water heating worth it?

In sunnier countries like Greece, solar water heating is highly effective year-round. However, the UK climate presents challenges, with shorter daylight hours and more overcast days, particularly in winter. 

Solar thermal systems are generally effective in the UK from April to September, providing up to 60% of a household’s annual hot water needs. However, a backup system (usually a gas or electric boiler) is needed to meet demands in the colder months.

Although solar thermal systems do help reduce carbon emissions and are a step toward sustainable energy use, their high upfront costs, combined with long payback times, mean they may not be the most cost-effective option for many UK households. In practice, it can take well over a decade to recoup the initial investment, depending on household hot water usage and energy savings.

Given these factors, we believe that investing in solar PV panels paired with a heat pump system and battery storage offers a more flexible and financially viable alternative over the long term. 

Solar PV systems can power electric heating solutions, such as air-to-water heat pumps, which provide year-round heating and hot water. Additionally, battery storage allows excess solar electricity to be stored for later use, helping to further reduce reliance on the grid and maximise efficiency.