Solar Thermal: Complete Guide to the Pros, Cons and Costs

Solar thermal panels efficiently convert sunlight into energy, using it to create hot water for your home. 

Solar thermal is an older technology than solar photovoltaic (PV) panels, and while the latter has seen huge growth in the last decade – in no small part thanks to the now-finished Feed-In Tariff (FiT), which provided generous payments to homeowners – there’s still a place at the table for solar thermal panels, depending on your property's needs. 

If you’re considering this type of solar panel for your home, this complete guide sets out exactly what you need to know before you invest. 

What are Solar Thermal Panels?

In a nutshell, solar thermal panels create heat for use in domestic hot water. (By comparison, solar PV panels convert sunlight into electricity.) In the summer months, solar thermal panels could meet all or a substantial proportion of your domestic hot water demands. It is a simple, reliable technology which comes with a number of benefits.

However, installing solar panels is not a one size fits all solution, and weighing up whether solar thermal panels are right for your home will depend on factors. 

The cost of solar panels, how much hot water you use, the orientation of your roof, and your existing fuel source will all impact the decision-making process.

How Does Solar Thermal Work?

Thermal solar panels work as sunlight passes through a panel and is refracted by the glass; this changes its wavelength, essentially trapping it and producing heat. The heat is captured in a fluid and conveyed to a hot water cylinder.

In the UK, there is a closed loop of fluid between the panel and the storage cylinder that contains anti-freeze — generally a 50/50 water/glycol mix. This means the fluid in the panels never actually reaches the taps in the home. This is known as an indirect system. 

There is a small pump that circulates fluid and it is usually triggered by a temperature sensor that will operate the pump (and move the fluid to the cylinder) if the panel is at least four degrees warmer than the temperature of the water in the storage cylinder. 

Indirect systems typically feature a twin coil storage cylinder. It's also worth noting that the cylinder needs to be bigger than the standard cylinder — typically 200 to 350 litres. A large hot water storage cylinder allows the system to retain as much heat as possible whilst the sun is shining.

What Different Types of Solar Thermal Panels are Available?

There are two types of solar thermal panels available for domestic properties: flat panels and evacuated tube solar thermal panels.

• The flat panel: The most common type of solar thermal is a flat panel (also known as a collector), usually around 1m x 2m in area. Each panel contains a series of pipes that are either serpentine or grid shaped, with a metal (absorber) plate fixed on top that is coated in a highly absorptive blueish material (selective coating). The metal absorber plate collects heat from the sun; the fluid in the pipes then carries this heat to a storage cylinder in the house. The panels can be installed on a roof in a landscape or portrait configuration.

• Evacuated tube: These are glass tubes that have a copper tube within them. Between 10 and 30 of them are connected together with a header pipe to form a panel. Evacuated tubes have a vacuum in the glass that acts as the insulation and so are often a bit more efficient than flat panel solar collectors. 

Some evacuated tubes known as ‘direct flow’ can be installed very discreetly on a flat roof, which could be particularly useful in sensitive areas such as conservation areas or on listed buildings (subject to planning permission).

Hybrid solar panels, also known as solar PVT, combine the technologies of solar PV and solar thermal into one system. 

How Much do Solar Thermal Panels Cost?

Installing a two or three panel solar thermal system that would supply an average 200 to 300 litre cylinder will cost around £4,000 to £7,000. 

The cost of solar panels can vary according to the complexity of the pipe runs and roofing materials, and you would also expect to be at the higher end of that scale if using evacuated tubes.

You'll also need to factor in the cost of a twin coil hot water cylinder, which cost from £600 to just under £2,000, depending on the size and specification.

How Much Water Can Solar Thermal Panels Heat?

"A solar thermal system will heat any quantity of water as long as the amount of panels is sufficient," says John Gilham, group technical manager at Green Building Renewables. "They work best when the system is designed to meet the output."

If you're asking 'can you run a house on solar power?', the answer is, to an extent with regards to solar thermal. The percentage of the hot water requirement that is provided by the solar panel is known as your solar fraction and it is by limiting the use of the back-up boiler that you can increase the solar fraction and maximise the return on investment.

For example, if you have a shower in the evening do you then allow the boiler to
reheat the water that you have used or do you leave the water in the cylinder and just size the capacity of the cylinder to provide enough hot water for the whole household?

In a small household it is easier to manage but in a larger home there may not be
enough capacity and you may need the boiler to reheat some water. This means
that the solar panel will only heat some of the water in the cylinder the following day,
thus lowering the solar fraction.

What the Advantages and Disadvantages of Solar Thermal?

For many people the financial payback on these systems isn’t very good, especially if you only have two people living in a four bed home. However, as they are fairly low tech products the carbon emission savings can be very beneficial. So for self builders with a planning condition that requires a percentage of your energy to come from renewables, solar thermal can be a great solution. 

What’s more, in a new build eco house you may already be installing a hot water cylinder as part of your heating and hot water system, so the real extra cost for solar thermal is lower as you would need the cylinder anyway, and the scaffolding and plumbers may already be on site which again lowers the installation costs. The picture is not as sunny for most renovators, though. 

That said, now that there is once again a level playing field for renewables (in the absence of the Feed-in Tariff, which provided a generous payment to owners of solar PV) we will definitely see more solar thermal solutions becoming available. What's more, you'll no longer be paying VAT on purchasing solar thermal panels.

They take up less space on the roof than solar PV and also have virtually no running costs. It is only really the amount of hot water that you use and your heat source that will define the return on investment, as these products really do work and should be on the radar when considering alternative technologies.

Can Solar Thermal Be Used For Central Heating? 

Solar thermal systems are only really suitable for domestic hot water preparation and are seldom suited to central heating applications. 

Sunlight as a resource is too low in winter, while on the other hand you could end up with huge over-generation in summer. 

If you are running a large thermal store or combining with a biomass boiler and system, there may be times when an oversized solar thermal array could be beneficial, but careful design is required, as the panels will be sized according to the volume of the thermal store rather than household usage.

Can You Use Solar Thermal with a Combi Boiler?

You can't effectively use solar thermal panels with a combi boiler. For the best use of solar thermal, a hot water store is required, meaning a system boiler configuration, which consists of a boiler and separate hot water cylinder, works best. If you have a combi boiler then you will not have a hot water cylinder. 

At best, the solar thermal system will only act as a pre-feed to the combi and will therefore have a very limited efficacy. A combi boiler can only realistically have an input temperature of up to around 29°C so, at most, the solar thermal system will only contribute around 20°C.

How Effective is Solar Thermal in Winter?

As the nights draw in you may be thinking 'do solar panels work in winter?' and the answer is yes, but not as efficiently as in the summer.

"You can still have solar thermal gain during winter on sunny winter days," explains John Gillham. "However, solar thermal is affected by the loss of direct sunlight a bit more than PV, and realistically you should not expect too much from your solar thermal during winter. They are also affected by clouds considerably more than solar PV."

The frequency of sub-zero temperatures also means that the panels need to be protected from freezing. It is therefore necessary to have a closed loop of fluid between the panel and the storage cylinder that contains anti-freeze. (All systems installed in the UK have this as standard.)

Can You Install Solar Thermal Yourself? 

"Renewable technology installation is a skilled and specialist job, and knowledge of electric and heating systems is required," advises John Gilham. "If you wouldn't feel comfortable installing a boiler or electrics into your home, then it's highly recommended you do not try to install your renewable technology either. 

"A trained specialist with the necessary regulations and safety knowledge can ensure that a solar thermal system is installed correctly, safely and efficiently for your home. Solar thermal installation often requires works at height on roofs, and this should never be conducted without the necessary safety precautions."

The work also falls under Building Regulations, meaning you'll need to either:

• Apply for Building Regulations sign off from your local authority building control department or from a private Approved Inspector
• Or, follow the more common route of opting for a company or individual that belongs to the Competent Person Scheme and can 'self certify' the work.

Do Solar Thermal Panels Need Planning Permission?

Installing solar thermal panels now typically falls under Permitted Development, meaning you do not require planning permission for solar panels. 

However, there are caveats to this — the panels must not protrude more than 150mm off the profile of the roof and must not be higher than the highest part of the roof (excluding the chimney), for instance.

There are also exceptions, notably conservation areas and installing solar thermal panels on or in the vicinity of a listed building will require consent. If in doubt, speak to the local authority.

How Long Do Solar Thermal Panels Last?

Solar thermal panels can last as long as 25 years, but generally come with a 5-10 year warranty. For this duration solar thermal panels also require very little maintenance, although check this with your supplier. 

How Much Money Could I Save by Installing a Solar Thermal System?

Savings will depend on your hot water usage, the system design, and the fuel you are replacing (gas, electric, LPG, for instance). 

For a four-bed house you would likely use around 200 litres of hot water per day, 365 days a year. 

That would mean that you actually use around 12kWh of energy per day on hot water. If we assume that half your annual hot water comes from solar then this equates to 2,184kWh per year (12kWh x 182 days) of 'free' energy. Let's compare that to the cost of producing the same energy using gas and electric:

• Gas: 2,184kWh per year x 7p (average) = £152.88
• Direct electricity: 2,184kWh per year x 28p (average) = £611.52

A saving of around £150 per year would give us a payback period of around 26 years on the capital cost of installing a solar thermal system, whilst a saving of circa £600 would give us a payback of just under 7 years. (However, this does not take into account off-peak electricity tariffs / unit prices, which would be much lower at night.)

The real potential of solar thermal comes when you design it to give you more than 60% of your annual hot water and make sure that the system is not oversized. 

It's worth noting that grants for solar panels such as the Renewable Heat Incentive initiative, which paid out a deemed amount for seven years based on your deemed hot water usage, have come to an end. 

The RHI ended in March 2022 and the Green Homes Grant was closed for new application in March 2021. 

Is Solar Thermal Worth It?

"The popularity of solar thermal panels has decreased due to the costs. The ending of the Renewable Heating Incentive (RHI) has also impacted the uptake of solar thermal. However, when installed for the correct application, it can still be a very effective renewable technology for homes and commercial buildings," explains John Gilham.

"The crucial thing to consider when it comes to solar thermal systems is that they are designed so that the hot water demand is approximately equal to the output of the panels installed. Otherwise, the system can overheat as it cannot expel the heat. With this in mind, solar thermal systems work exceptionally well where there is high hot water usage, especially during the middle of the day, such as in swimming pools and commercial applications. 

"In contrast, if you have minimal water usage, a solar thermal system is probably not cost-effective for domestic hot water use. If you are using solar thermal to replace an oil or LPG heating system, then savings will be more significant, especially as the per-unit fuel cost continues to rise. For households on electric off-peak heating systems, savings will be more modest. 

"The savings for replacing a gas system are more effective when it is a larger household with more people using hot water."

On a final note, while payback may be slow, there are other benefits. Solar thermal should offer an inflation-proof option for your home's future hot water and offer better savings as the cost of alternative energy sources increases.