Solar thermal panels use heat from the sun to help provide hot water.
They are different from solar PV panels.
Solar PV makes electricity.
Solar thermal heats water.
So, if you are considering solar thermal, the first thing to understand is what it can — and cannot — do.
How Does Solar Thermal Work?
Solar thermal collectors are fitted outside, usually on the roof.
A fluid flows through the collectors and is warmed by the sun.
It then carries this heat to a hot-water cylinder inside the home.
In simple terms:
Sun → Solar collector → Hot-water cylinder → Taps and showers
The fluid in the solar system does not normally come out of your taps.
Instead, it passes through a coil inside the cylinder and transfers its heat to the stored water.
Then, the fluid returns to the collectors and the process starts again.
Is It the Same as Solar PV?
No.
Although both use energy from the sun, they do different jobs.
Solar PV panels → make electricity
Solar thermal collectors → provide heat for hot water
So, solar thermal will not normally power your lights, appliances or other electrical equipment.
If you want to make electricity, you need solar PV.
Learn more: Solar PV and Battery Storage →
What Does the System Need?
A typical solar thermal system has several main parts:
- solar collectors
- pipework
- a pump
- controls
- a suitable hot-water cylinder
The collectors gather heat.
Then, the pump moves the heated fluid between the collectors and the cylinder.
The controls decide when the pump should run.
Does It Need a Hot-Water Cylinder?
Usually, yes.
The heat collected during the day needs somewhere to go.
So, solar thermal normally works with a hot-water cylinder that can accept heat from the solar system.
This is important if you currently have a combi boiler and no cylinder.
Adding solar thermal could mean finding space for a suitable cylinder as well as installing the collectors.
Learn more: Your Hot Water System →
Does It Work All Year?
Solar thermal can collect heat throughout the year.
However, the amount changes with the seasons and weather.
Generally, it can provide more useful heat during sunnier months and less during winter.
So, you should not normally expect solar thermal to provide all your hot water throughout the year.
Another heat source will usually still be needed.
This could be:
- a boiler
- a heat pump
- an immersion heater
Solar thermal helps reduce the amount of energy that this main system needs to provide.
What Happens in Summer?
Solar thermal can work particularly well when there is plenty of sunshine.
At times, it may provide much of the hot water a household needs.
However, the system still needs to deal safely with times when plenty of heat is available but little hot water is being used.
For example, this could happen while you are away.
So, good system design matters.
What Happens in Winter?
There is less useful solar energy during winter.
Days are shorter and the sun is lower in the sky.
As a result, your boiler, heat pump or immersion heater is likely to do more of the work.
This seasonal difference is important when looking at expected savings.
Do not judge the system only by its summer performance.
Does Roof Direction Matter?
Yes.
The direction, angle and position of the collectors can affect how much useful heat they collect.
Shade also matters.
For example, nearby:
- trees
- buildings
- chimneys
may reduce the amount of sunlight reaching the collectors.
However, a roof does not necessarily have to face perfectly south.
So, the useful question is not simply:
“Does my roof face south?”
Instead ask:
“How much useful heat could this system collect on my roof?”
How Much Hot Water Can It Provide?
There is no single percentage that applies to every home.
It depends on things such as:
- collector size
- collector position
- shade
- weather
- hot-water use
- cylinder size
- system design
Also, the result changes through the year.
So, be careful with claims that a solar thermal system will always provide a fixed percentage of your hot water.
Ask for an estimate based on your property and your hot-water use.
Will It Heat My Radiators?
Most domestic solar thermal systems are mainly designed to help provide hot water.
Using solar thermal to provide meaningful home heating is more difficult because the greatest need for space heating is usually during the colder, darker part of the year.
That is also when less solar heat is available.
So, for most homeowners, it is better to think of solar thermal primarily as a hot-water system.
Will It Save Money?
It can reduce the amount of energy needed from your main hot-water system.
However, that does not automatically mean it will be good value.
The financial result depends on:
- installation cost
- amount of useful solar heat
- hot-water use
- the energy source being replaced
- energy prices
- maintenance costs
For example, replacing expensive electrically heated water could have a different financial result from reducing the amount of gas used by an efficient boiler.
So, compare the expected saving with the full cost of installation.
Solar Thermal or Solar PV?
This is an important comparison because both systems compete for roof space and money.
Solar thermal makes useful heat directly.
Solar PV makes electricity.
That electricity can be used for many things around the home. It can also sometimes be stored in a battery or exported to the grid.
Therefore, do not choose solar thermal simply because you want to “use solar”.
Instead, compare what each system would actually do for your home.
Consider:
- available roof space
- electricity use
- hot-water use
- existing heating system
- installation cost
- expected savings
Then decide which use of the roof gives you the most useful result.
What Maintenance Is Needed?
Solar thermal systems contain more than just roof collectors.
They also have fluid, pumps, controls and other parts.
So, some checks and maintenance may be needed over the life of the system.
Before buying, ask:
- what needs checking
- how often servicing is recommended
- how the system fluid is maintained
- what the warranty covers
- what parts may eventually need replacing
Also, make sure you know who can service the system in the future.
Before You Buy
First, look at how much hot water your household uses.
Then, check your existing hot-water system.
Next, find out whether your roof has enough suitable, unshaded space.
Also ask:
- how much useful heat the system is expected to provide
- how that estimate was calculated
- what cylinder is needed
- whether your existing cylinder can be used
- what happens when solar heat is not enough
- what happens when there is more heat than you need
- what maintenance is required
- what warranties apply
- what the full installation will cost
Finally, compare solar thermal with the other options available to you.
Do not compare technologies only by efficiency.
Compare what they provide, what they cost and how well they suit your home.
Key Points
- Solar thermal uses heat from the sun to help provide hot water.
- Solar thermal and solar PV are different. Solar thermal provides heat for hot water, while solar PV makes electricity.
- A suitable hot-water cylinder is usually needed to store the heat collected by the system.
- Solar thermal works throughout the year, but output changes with the seasons. It normally provides more useful heat in summer and less in winter.
- You will usually still need another way to heat your water, such as a boiler, heat pump or immersion heater.
- Your roof matters. Direction, angle and shade can all affect how much useful heat the collectors provide.
- There is no single percentage of hot water that solar thermal will provide. This depends on the system, property, weather and how much hot water you use.
- Solar thermal is mainly used for hot water in homes. It is generally less suited to providing significant space heating.
- Lower energy use does not automatically mean good value. Compare the expected saving with the full installation and maintenance costs.
- Compare solar thermal with solar PV before using valuable roof space. They provide different benefits, so consider which would be more useful for your home.
- Maintenance may be needed. Pumps, controls, system fluid and other parts should be considered as well as the roof collectors.
- Get an estimate based on your own property and hot-water use rather than relying on general savings claims.
- Most importantly, compare what the system will provide, what it will cost and how well it suits your home before you buy.
Important Information
Energility provides general information to help you understand energy, household and business costs, services and ways to save money. It is not financial, legal, tax, technical or professional advice.
Prices, tariffs, grants, government schemes, regulations and other information can change. Costs, savings and calculator results are estimates and your actual results may be different.
Before making an important financial decision, signing a contract or carrying out significant work, check the latest information and consider getting advice from a suitably qualified professional where needed.
Energility may receive a commission or other benefit from some links or services. Where this applies, we aim to make it clear. This does not increase the price you pay unless stated otherwise.
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