A beginners guide to heat batteries

If you’ve started looking into ways to make your home’s hot water more efficient, you may have come across the term “heat battery”. Unlike a conventional battery, a heat battery doesn’t store electricity. Instead, it stores thermal energy (heat) and releases it when you need it.

Heat batteries are becoming an increasingly popular alternative to traditional hot water cylinders, particularly in homes with solar panels, heat pumps, or access to cheaper off-peak electricity. But how exactly do they work, and could one be right for your home?

What is a heat battery?

A heat battery is a device designed to store heat and release it later when hot water is needed. Think of it as a thermal version of a rechargeable battery.

A conventional hot water cylinder stores energy by heating a large volume of water. A heat battery takes a different approach: it uses a specialised material that can absorb and release a large amount of heat as it changes state.

This allows a heat battery to store a significant amount of thermal energy in a much smaller space than a traditional hot water cylinder.

Sunamp’s Thermino heat batteries use a high-performance phase change material (PCM) called Plentigrade. This material stores heat as it changes between solid and liquid, then releases that stored energy when hot water is required.

How does a heat battery work?

To understand a heat battery, it helps to understand the idea of phase change. Water can exist as a solid, liquid or gas. When it changes between these states, energy is absorbed or released. A phase change material works in a similar way.

When a Sunamp heat battery is being charged, heat is transferred into the phase change material, causing it to change state and store thermal energy. When you turn on a hot tap, the process works in reverse. The stored heat is released and transferred to the incoming mains water, providing hot water when you need it.

Sunamp’s Plentigrade PCM is specifically engineered for thermal storage, and can store significantly more useful energy than water over the same temperature range. This is one of the reasons a Thermino heat battery can be up to four times smaller than the conventional hot water cylinder it replaces.

In simple terms, it is essentially a rechargeable store of heat.

How is a heat battery different from a hot water cylinder?

The biggest difference is what is actually storing the energy. A traditional hot water cylinder stores a large volume of hot water. As the water cools, the amount of useful hot water available gradually decreases. A heat battery stores thermal energy within its phase material instead.

This means you don’t need a huge tank full of hot water sitting in your home. Sunamp’s Thermino heat batteries can be up to four times smaller than the equivalent conventional hot water cylinder, while still providing high flow rate, mains pressure hot water.

This can be particularly useful in smaller homes, apartments, and properties where cupboard or utility room space is limited.

Does a heat battery store hot water?

No. This is one of the most important things to understand about a heat battery.

A Sunamp Thermino doesn’t work like a conventional hot water cylinder, where hundreds of litres of water are stored inside a tank. Instead, the heat battery stores thermal energy. When you open a hot tap, mains water passes through the heat battery’s heat exchanger and is heated using the stored energy. The result? Hot water on demand, without needing to keep a large volume of stored water inside a conventional cylinder.

Where does the heat come from?

This depends on the particular heat battery model. Different Thermino models are designed to work with different heat sources, including grid electricity, solar PV, heat pumps, and boilers.

As an example, the Thermino ePlus uses an integrated electrical heating element, while other Thermino configurations can be charged from external heat sources such as boilers or heat pumps. Some models can also be configured to make use of electricity generated by a solar PV system.

This flexibility is one of the reasons heat batteries can work particularly well as part of a wider home energy system.

Why pair a heat battery with solar panels?

Solar panels generate electricity when the sun is shining, but your household may not always need that electricity at the exact moment it is generated. Instead of allowing surplus solar electricity to go to waste or exporting it to the grid, you can use it to produce and store heat.

A heat battery can therefore act as another way of storing your solar energy. Your solar panels generate electricity, and any surplus charges the heat battery. The stored heat is used for hot water whenever you need it, even if the sun isn’t shining. This can be particularly useful for households that want to make greater use of the electricity their solar panels generate themselves.

Can a heat battery use cheap electricity?

Yes. One of the major advantages of thermal storage is that you don’t necessarily need to produce heat at the same time that you use your hot water. If you have a time-of-use electricity tariff with cheaper overnight or off-peak periods, a heat battery can be charged when electricity is cheaper and then provide hot water throughout the day. This is similar in principle to charging an electrical battery overnight and using the stored energy later.

Sunamp highlights this ability to shift hot water production to lower-cost periods as one of the key benefits of its heat battery technology.

What are the advantages of a heat battery?

There are several reasons homeowners are considering heat batteries as an alternative to traditional hot water cylinders.

1.      They take up less space

Because phase change materials can store a large amount of thermal energy in a relatively small volume, heat batteries can be considerably more compact than conventional cylinders. Sunamp’s Thermino range is designed to be up to four times smaller than the equivalent hot water cylinder.

2.      They can provide mains-pressure hot water

Thermino heat batteries are designed to provide high flow rate hot water directly from the mains, rather than relying on a large tank of stored hot water.

3.      They can work with renewable energy

Depending on the model and installation, heat batteries can work alongside solar PV and heat pumps, helping households make better use of renewable or lower-cost electricity.

4.      They can take advantage of cheaper electricity

A heat battery allows you to produce and store heat when electricity is cheaper, rather than necessarily heating water at the point of use.

5.      Very little heat is lost while stored

Thermino heat batteries use high-performance insulation to minimise standing heat losses.

6.      Thery don’t require a large tank of stored hot water

For homeowners who don’t have space for a conventional cylinder, this can make a significant difference.

Are heat batteries safe?

Sunamp Thermino heat batteries are designed specifically for domestic hot water applications and use a non-toxic, non-flammable phase change material. The technology is also designed to reduce the risk of Legionella associated with traditional stored hot water systems.

As with any hot water system, installation should be carried out by a suitably competent professional and in accordance with manufacturer’s installation requirements, as well as relevant UK regulations.

How long does a heat battery last?

A common question when people first hear about heat batteries is whether the repeated charging and discharging will eventually wear the unit out. Sunamp’s Plentigrade technology has undergone extensive cycle testing. Sunamp reports no noticeable degradation up to 10,000 cycles, with testing demonstrating failsafe performance beyond 40,000 cycles.

The expected lifespan will depend on the specific model, installation, and how it is used, but the technology is designed for long-term everyday operation.

What size heat battery do I need?

Heat batteries come in different sizes, and choosing the right one depends primarily on your household’s hot water requirements. The current Thermino range includes models with different capabilities, including the 70, 150, 210 and 300 sizes. These numbers are expressed as litre-equivalent sizes to make comparison with conventional hot water cylinders easier. For example, a Thermino 210 is broadly intended to provide the hot water capacity you may associate with a 210-litre cylinder, despite being considerably more compact.

The correct size depends on factors including the household occupancy, number of bathrooms, and bath usage. For a more detailed explanation, please see our guide What size Sunamp do I need?

Is a heat battery the same as an electrical battery?

No. Although the concept is similar, they store different types of energy. An electrical battery, such as Tesla Powerwall, stores electricity. A heat battery stores thermal energy.

The two technologies can work particularly well together. A home with solar panels, an electrical battery, and a heat battery could potentially store surplus solar electricity in the electrical battery while also using some surplus generation to produce and store hot water.

Is a heat battery right for my home?

A heat battery can be particularly interesting if you don’t have enough space for a conventional hot water cylinder, you’re replacing an immersion heater, you have solar panels, you have a heat pump, or you have access to an off-peak electricity tariff. However, the best solution depends on the property and existing heating system.

The important thing is to choose the right heat battery and configuration for your heat source and hot water requirements.

So, is a heat battery worth considering?

For many homes, a heat battery offers an interesting alternative to the traditional hot water cylinder. The basic idea is actually very simple:

Use energy when it’s available or cheap, store it as heat, and use that stored heat when you need hot water.

The clever part is the phase change material that allows a large amount of thermal energy to be stored in a relatively small space. Sunamp’s Thermino range takes this principle and packages it into a compact hot water solution designed for modern homes, with compatibility across different energy sources and systems.

If you’re considering a heat battery, the next step is to work out which Thermino model is right for your home, and how it could integrate with your existing heating, hot water, and energy system.