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Solar Thermal: Pros, Cons, Costs and More

a vacuum solar thermal panel in a garden
(Image credit: getty images)

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

Solar thermal is specifically used for creating hot water for your home, and is particularly efficient at converting sunlight into energy. 

Interest in solar thermal technology has had a boost in recent times. This type of solar panel qualifies for the Green Homes Grant as a primary measure, allowing you to claim vouchers against the cost on installation of up to £5,000. 

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

What is Solar Thermal?

Solar thermal is a system using panels to create heat for use in domestic hot water. 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, it’s not a one size fits all solution, and weighing up whether solar thermal panels are right for your home will depend on factors such as how much hot water you use, the orientation of your roof, and your existing fuel source.

What is the Difference Between Solar Thermal and Solar PV Panels?

Solar thermal converts sunlight into heat for hot water, while solar PV panels convert this same energy into electricity. 

A solar thermal panel is around 80% efficient, compared to approximately 20% for solar PV,  meaning you’ll likely need a lot less of them on the roof.

Now that there is once again a level playing field for renewables (in the absence of the Feed-in Tariff for solar PV), and with the introduction of the Green Homes Grant, we should see more solar thermal solutions becoming available. 

How Does Solar Thermal Work?

What is a twin coil cylinder?

A twin coil cylinder or thermal store features a coil of pipework at the bottom that is connected to the solar panel, and a second coil of pipework further up the cylinder that is plumbed to the boiler (which provides a backup on those days when there is less sunlight). 

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 cylinder allows the system to store as much heat as possible whilst the sun is shining.

(MORE: Hot Water Storage)

What are the Types of Solar Thermal Panels?

There are two types of solar thermal panels available for domestic properties:

flat solar thermal panel on roof

(Image credit: getty images)
  • 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 solar thermal panels on a rooftop

(Image credit: getty images)
  • 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.

What are the Advantages of Solar Thermal?

  • They take up less space on the roof than solar PV panels
  • There are virtually no/minimal running costs
  • Simple, reliable technology which can lower your energy bills.
  • Eligible for Green Homes Grant and Renewable Heat Incentive (RHI)

Disadvantages of Solar Thermal

  • Weather dependent, and less efficient during winter months
  • The payback period can be some 20 or so years, depending on your existing fuel source and hot water use.
  • Can only generate hot water, not electricity. 

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 costs 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.

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 5p (average) = £109
  • Direct electricity: 2,184kWh per year x 15p (average) = £327.60

A saving of £109-£327 per year would give us a payback period of around 20 years on the capital cost of installing a solar thermal system.

So, 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. 

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.

In addition, the Renewable Heat Incentive initiative, which pays out a deemed amount for seven years based on your deemed hot water usage, helps matters here.

The RHI payment could be around £300 to £400 for a three to four-bed house. That could then contribute to around half or more of the capital cost over time. The domestic RHI is currently road mapped to end on 31 March 2022.

And finally, the Green Homes Grant (GHG) could contribute quite significantly towards the capital cost. The government will pay at least two-thirds of the cost of installing a solar thermal system, up to £5,000, under the grant.  The GHG is currently scheduled to end on 31 March 2022 also. 

As such, the case for installing a well-designed solar thermal system, on the right property, is growing.

Is Solar Thermal Worth It?

The financial payback on these systems is often not very good, especially if you only have two people living in a four bed home. 

However, 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 self build 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. You could of course also get the VAT back. 

It is only really the amount of hot water that you use and the heat source you would use otherwise that will determine the return on an investment in solar thermal panels, but they should definitely be on the radar when considering alternative technologies.

Do I Need Planning Permission to Install Solar Thermal Panels?

Installing solar thermal panels now typically falls under permitted development, meaning you do not require planning consent. 

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.

The work does however fall 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.

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 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 Solar Thermal Panels Work with a Combi Boiler?

Not effectively, no. 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.

Can You Get Discreet Solar Thermal Panels?

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).

David is a renewables and ventilation installer, with over 35 years experience, and is a long-standing contributor to Homebuilding and Renovating magazine. He is a member of the Gas Safe Register, has a Masters degree in Sustainable Architecture, and is an authority in sustainable building and energy efficiency, with extensive knowledge in building fabrics, heat recovery ventilation, renewables, and also conventional heating systems. He is also a speaker at the Homebuilding & Renovating Show. 

Passionate about healthy, efficient homes, he is director of Heat and Energy Ltd. He works with architects, builders, self builders and renovators, and designs and project manages the installation of ventilation and heating systems to achieve the most energy efficient and cost effective outcome for every home.