Solar Thermal Panels: Everything you Need to Know

Find out all you need to know about solar thermal panels in this ultimate guide.


Solar Panels cover a couple of different technologies: Solar Photovoltaic panels (PV) are used for generating electricity, whilst Solar Thermal panels are used to provide domestic hot water.

Solar Thermal Panels

Solar Thermal Panels are used to provide domestic hot water for showering, washing dishes, and anything else you might need it for. The panels harness the sun’s energy and use it to heat water stored in a hot water cylinder. Often, a boiler or immersion heater is used to generate additional warm water when the panels cannot keep up with demand.

Types of Solar Thermal

There are two kinds of solar thermal panels. Evacuated tube solar panels are fitted above the roof, laid on top of the existing slate or tile covering. Flat plate collectors can also be fitted this way, but may otherwise be integrated into the roof covering itself, resulting in a more streamlined appearance.

Evacuated Tube Solar Panels

The Pros and Cons of Evacuated Tube Panels

+ Very good heat retention compared to flat plate collectors.

+ Easily transported and fitted as each tube can be handled separately.

+ Can operate well with the sun hitting them at any angle.

+ Able to work in low temperatures or places with a below-average solar source, such as the UK.

- These panels will cost up to twice as much as flat plate collectors.

- Evacauted tubes will reach a maximum of just  50% efficiency.

Flat Plate Collectors

The Pros and Cons of Flat Plates

+ Flat plate collectors are quite inexpensive when compared to evacuated tubes.

+ The panels can run at up to 75% efficiency.

+ The heat of panels is able to melt snow and ice before it really sets.

+ Flat plates are the most common, and most well-tested kind of solar thermal panel.

+ CO2 Emissions are lower than those of evacuated tube systems.

- The copper used to make flat plate panels varies in cost, meaning that the cost of the panels can vary greatly in turn.

- Flat plate panels must be positioned where they are most directly facing the sun as sunlight is quite easily reflected from the panels’ smooth surface.

Active vs Passive Panels

Solar thermal panels are available as parts of both active and passive systems. Which is best for you will depend on how you wish to power your system as well as the climate where you live.

Active Solar Thermal Panels

Active solar water heating systems use electrical pumps, valves, and controllers to circulate water or other fluids through their pipes.

1. Direct (open) Circulatory Systems. 

These utilise pumps to push water through the panels, and are best sited to places where little to no freezing is likely to occur and where the water is neither hard nor acidic.

2. Indirect (closed) Circulatory Systems. 

These utilise pumps to push other fluids such as glycol and water mixes around the panels. The system requires a heat exchanger to transfer eat from the fluid to the domestic hot water tank.

3. Drainback Systems. 

These are a kind of indirect system which use only water in the pipes. The water in the top loop is stored in a tank, and heat is transferred to the domestic water supply. These are particularly useful in colder climates.

Passive Solar Thermal Panels

Passive systems avoid using pumps to push water through the solar thermal system, and are usually less expensive. Gravity and the natural motion of heated water do all of the work, and the systems use no electricity, require little maintenance, and can last much longer. However, they are generally less efficient.

1. Integral Collector Storage Systems. 

These systems use storage tanks housed within insulated boxes to draw the sun’s energy from a glazed wall facing the sun. During the winter, these systems must be drained to avoid freezing, and the systems will lose energy overnight. Integral collector systems are best used where temperatures are unlikely to fall below freezing.

2. Thermosyphon Systems.

These systems utilise warm water’s natural convection to move water within the panels and to the tank located above the collector. The water rises as it heats and is stored within the tank. To prompt circulation, cool water is passed through the pipes to the bottom of the panel. Using glycol fluid in the water can mean the systems are able to work even in freezing conditions, as long as the space is sufficiently protected.

Preparing for Solar Thermal Panels

Sizing the System

The necessary size of your solar thermal system will depend entirely upon the amount of domestic hot water you and your household will require. Current building regulations state that the hot water store used to supply the system must provide no less than 80% of the daily hot water demands, or 25 litres per square metre of the collector area.

When designing a system, you will need to configure a setup which will not generate more energy than is needed to recharge the hot water store in times of low demand, such as the summer. Using a large store can result in having low temperatures in times of very low solar availability, thus wasting money and space and not providing the water needed.

In cases where a system is beginning designed for a new structure, the best way to determine the necessary size is to consider the water consumption of buildings of a similar size. For retrofitted systems to be installed in existing buildings, you will need to have your demands measured in the warmer summer months. The determination of your system requirements is an important process, and should be undertaken by someone with significant and relevant experience in the process of solar thermal system design.

Backup Heat Sources

At times, your solar thermal system may be unable to supply the amount of domestic hot water you need. To make sure that there is still enough hot water available, it is always wise to have fitted a backup system to compensate. A conventional water cylinder will work well, and can be used to keep warm water in continuous flow.


In order to keep the water moving, you will need to use pumps. The energy that is needed to keep the water moving can be great, so it is a god idea to keep pressure drops within the system as low as possible with some careful pipework planning and the use of solar powered pumps. An experienced professional will be much better qualified to do this than most people, and can make sure that you get the best out of your solar thermal panels.


Solar thermal system controllers should be chosen by taking into consideration the following points:

1. How much control do you need?

Different control systems will offer different levels of control. It goes without saying that the more you pay, the more control features you can expect to get.

2. User Interface

Pick a user interface you feel comfortable using. You will usually find displayed information such as the temperature at the top of the tank, the pump speed, and a count of the time the pump has been in operation.

3. Energy Usage

The control is also able to measure heat quantity so that you are able to see how much heat the system has collected. It is also able to use additional sensors to calculate and show you how much energy input is supplied to the storage tank and will display a cumulative sum of the Kwh of energy collected by the solar thermal panels.

4. You Get what you Pay For

It is generally advisable to spend a little more on such an important part of the system, even though opting for a cheaper model may be tempting. More functions will make it easier to ensure your system is running efficiently, and choosing an MCS accredited piece of equipment will ensure that the kit you buy is reliable and up to the job.

Small Systems

For most people who are looking to install a solar thermal system to supply their domestic hot water demands, the most crucial issue to overcome is that of roof space. Often, on older, smaller, or terraced houses, the space available can be very limited, leaving little room for any kind of solar array to be installed. In many cases, the sensible option is to go for evacuated tube solar panels as they will achieve greater outputs using less space. Designing a small system is absolutely possible, but for those really pushed for available space, it is possible to find wall-mounted, A-frame-mounted, and even ground-laid panels. However, these will not work as efficiently, are more susceptible to damage and gathering debris, and can easily be shaded by trees or neighbouring buildings.

Fitting Solar Thermal Panels

Having completed all of the research and preparation, you can go ahead and have your solar panels installed. Installation is relatively simple for those in the know, but really is a no-go for the inexperienced. The main issue that could arise if trying to do this yourself is obvious: a collapsing roof. When working with such an important an integral part of the home, the best advice is to hire a professional.

Remember that when fitting flat plate collectors, the panels must be properly positioned so as to avoid reflecting all of the sunlight away.

Using Solar Thermal Panels

Once everything is in place, it’s time to put your new renewable energy system to use. Here’s how:


The best way to keep your solar panels working well is to properly maintain them. A well-looked-after system can last up to thirty years, providing a lifetime of hot water to your household at a good efficiency rate. Performance will slow, but a little upkeep goes a long way. It will help keep the panels running safely at peak efficiency, at low cost, whilst generating the most energy they can.

How to Maintain Solar Thermal Panels:

  • Check the system pressure every 1-2 months. It should be at between 1 and 2 bar.
  • Top up or replace the solar fluid every 5 years.
  • Keep the panels cleaned.
  • Fit the panels at a 45˚ angle to aid natural cleaning.
  • Keep an eye on the pump if you have one. These are the single moving component of any system, and are fairly inexpensive (around £100) to replace. Electronic controls may need replacing, but are also cheaply and easily replaced.
  • Watch out for leaks in the system, and if one is found then contact the system’s installer.
  • Try to keep track of performance. If something seems wrong, such as having no hot water when the sun is shining, get it checked.
  • Have an annual service check carried out by the installers.


 In order for your solar thermal panels to operate properly, you will need to have fitted a solar cylinder. Solar cylinders all feature a solar heat exchanger, the job of which is to allow the maximum heat transfer of solar energy into the water kept inside the cylinder. Solar cylinders are used with all kinds of systems, and provide your domestic hot water efficiently and effectively.

Direct Cylinders

These use an electric immersion heater to produce the additional hot water required when solar power is unable to keep up with high demand, as can often occur in colder, shorter, winter days.

Indirect Cylinders

These use traditional energy sources such as gas, oil, or electric boilers to generate the energy needed to provide the necessary additional domestic hot water.

Image by Julian

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