A Simple Guide to Using the Sun to Heat Water

Solar thermal uses energy from the sun to help provide hot water.

It is sometimes confused with solar PV. However, the two technologies do different things.

Solar PV generates electricity.

Solar thermal provides heat.

Solar thermal collectors absorb energy from sunlight. Then, a fluid carries that heat into your home.

The heat is transferred into water stored in a hot-water cylinder.

In simple terms:

Sunlight → Collector → Heated fluid → Hot-water cylinder → Hot water

Let’s follow the heat through the system.

It Starts on the Roof

Solar thermal collectors are usually fitted to the roof.

When sunlight reaches them, they absorb some of its energy as heat.

A special fluid flows through the collectors. As the collectors warm up, heat passes into this fluid.

The fluid then carries the heat towards the hot-water cylinder.

So far, the journey is:

Sunlight → Collector → Heated fluid

The fluid itself does not normally come out of your taps.

Instead, it transfers its heat to your stored water.

How Does the Heat Reach Your Water?

The heated fluid travels from the roof to a coil inside the hot-water cylinder.

This coil acts as a heat exchanger.

Heat passes through the metal coil and into the water around it.

However, the two fluids remain separate.

Once the solar fluid has given up some of its heat, it returns to the collectors.

Then, the process begins again.

Collect heat → Carry heat → Transfer heat → Return

That is the basic solar thermal cycle.

What Is in the Solar Pipes?

The fluid travelling through the collectors is different from your tap water.

It usually contains antifreeze protection because some of the pipework is outside.

A pump moves this fluid around the system.

Meanwhile, controls decide when circulation is useful.

For example, there is little point moving fluid from the roof if the collectors are colder than the water in the cylinder.

Therefore, the system monitors temperatures and operates the pump when useful heat is available.

Different Types of Solar Collector

Two types are commonly used.

Flat-Plate Collectors

These look rather like dark panels.

Pipework inside the collector carries the heat-transfer fluid.

They are relatively simple and widely used.

Evacuated-Tube Collectors

These use rows of glass tubes.

Their design helps reduce heat loss from the collector.

Although the two designs work differently, their purpose is the same:

Capture solar energy and turn it into useful heat.

Which type is suitable depends on the property and system design.

Why Do You Need a Hot-Water Cylinder?

The sun does not necessarily shine when you want a shower.

Therefore, the heat needs somewhere to go.

The hot-water cylinder provides storage.

Solar energy collected during the day can heat water that you use later.

There is a useful comparison here with the battery we discussed on the solar PV page.

A battery stores electrical energy.

A hot-water cylinder stores thermal energy.

They work in completely different ways.

However, both allow energy collected at one time to be used later.

What Happens When There Is Not Enough Sun?

Solar thermal will not normally provide all of a home’s hot water throughout the year.

During darker periods, less solar energy is available.

Therefore, another heat source usually works alongside it.

This could be:

• A boiler
• A heat pump
• An immersion heater

Suppose the solar system warms the water, but not enough.

The main heating system can then raise it to the required temperature.

In simple terms:

Solar provides what it can.

The main heating system provides the rest.

This reduces the amount of energy the main system needs to provide.

Summer and Winter Are Very Different

Solar thermal output changes throughout the year.

During bright summer weather, plenty of solar energy may be available.

At the same time, the home may need relatively little heating.

During winter, the opposite happens.

There is less solar energy available, while household heating needs are much greater.

Therefore, solar thermal is generally much more useful for hot water than for providing the main source of space heating.

This seasonal difference is important when considering what the technology can realistically achieve.

How Much Energy Does Hot Water Need?

This is where a little maths becomes useful.

Imagine we have:

150 litres of water

We want to raise its temperature from:

10°C to 50°C

That is a temperature increase of:

40°C

Ignoring heat losses, heating that amount of water requires roughly:

7 kWh of heat

That gives us something relatable.

If solar thermal provides some of those 7 kWh, the boiler, heat pump or immersion heater has less work to do.

That is the real purpose of the system.

It is not producing “free hot water” at all times.

Instead, it is reducing the amount of energy another system needs to provide.

Does Solar Thermal Work on Cloudy Days?

Solar thermal does not suddenly stop working because there are clouds.

Some solar energy can still reach the collectors.

However, less available solar energy usually means less useful heat.

Therefore, performance changes with the weather.

It is also affected by:

• Time of year
• Roof direction
• Collector angle
• Shading

A suitable roof with good access to sunlight will generally perform better than one that spends much of the day in shade.

What Happens When There Is Too Much Heat?

Summer can create the opposite problem.

Imagine a bright day when the collectors are producing plenty of heat.

However, the cylinder is already hot.

The system cannot simply keep heating the water indefinitely.

Therefore, solar thermal systems include controls and safety features to manage high temperatures.

This also explains why simply installing more collectors is not necessarily better.

The system should be matched to the home’s hot-water needs.

Solar Thermal or Solar PV?

This is an important question because both technologies may compete for the same roof space.

The difference is straightforward.

Solar PV

Turns sunlight into electricity.

That electricity can be used for:

• Appliances
• Battery charging
• EV charging
• Heat pumps
• Other electrical needs

Solar Thermal

Turns solar energy into heat.

Its main household use is helping to provide hot water.

Therefore, solar PV is more flexible.

However, that does not automatically make solar thermal unsuitable.

The right choice depends on the home and what you want the system to achieve.

Can You Have Both?

Yes.

A home can have both solar PV and solar thermal.

However, roof space is limited.

Therefore, it makes sense to consider how that space could best be used before installing either system.

For example, would the home benefit more from:

• Solar thermal for hot water?
• More solar PV?
• Solar PV with battery storage?
• Solar PV supporting a heat pump?

There is no universal answer.

The important thing is to look at the whole home, rather than choosing one technology in isolation.

Does Solar Thermal Need Maintenance?

Yes.

A solar thermal system contains more than roof collectors.

It may also include:

• Pump
• Controls
• Sensors
• Expansion vessel
• Heat-transfer fluid

These parts need to remain in good condition.

The heat-transfer fluid may also need checking or replacing over time.

Therefore, maintenance should be included when considering the long-term cost of the system.

Is Solar Thermal Right for Every Home?

No renewable technology is right for every property.

Solar thermal suitability depends on factors such as:

• Hot-water use
• Roof space
• Shading
• Existing heating system
• Installation cost
• Available alternatives

A household that uses plenty of hot water may have a different result from one that uses very little.

Likewise, a home planning to install solar PV and a heat pump may decide that its roof space could be used differently.

We will bring those choices together later in Is Renewable Energy Right for Your Home?

In Short

Solar thermal uses energy from the sun to help heat water.

Collectors on the roof absorb solar energy.

A fluid carries that heat into the home.

Then, a heat exchanger transfers the energy into water stored in a cylinder.

The journey is:

Sunlight → Collector → Heated fluid → Hot-water cylinder → Hot water

When plenty of solar energy is available, the main heating system has less work to do.

When solar energy is limited, the boiler, heat pump or immersion heater provides the remaining heat.

And the most important distinction is:

Solar PV produces electricity.

Solar thermal provides heat.

Once that difference is clear, solar thermal becomes a very simple technology to understand.

It collects heat when the sun provides it, stores that heat in hot water and reduces the amount of energy another heating system needs to supply.

Next: How Wind Turbines Work →

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