Storing energy

Home energy storage systems store generated electricity or heat, so that you can use the energy when you need it.

Electricity can be stored in electrical batteries, or it can be converted into heat and stored in a heat battery. Heat can also be stored in heat batteries or in thermal storage, such as a hot water cylinder.

Energy storage can be useful for people who generate their own renewable energy, as it allows them to use more of their low carbon energy.

Electrical batteries help you make the most of renewable electricity from solar photovoltaic (PV) panels, a wind turbine or a hydroelectricity system. For example, electricity generated during the day by solar PV panels could be stored in an electric battery for you to use for boiling the kettle or watching TV in the evening when your solar PV panels are no longer generating electricity.

The battery size and its cost will depend on your current energy use and the size of any generation technologies you have installed. You may also want to plan around future electricity use if you are intending to purchase an electric vehicle (EV) or heat your home with a heat pump.

There are two key types of heat storage: thermal stores and heat batteries.

Thermal stores are highly insulated water tanks that can store heat as hot water for several hours. They usually serve two or more functions:

• Provide hot water, just like a hot water cylinder.
• Store heat from a solar thermal system or wood-fuelled boiler, for use later in the day.
• Provide a ‘buffer’ function for heat pumps.
• Store heat from multiple sources, for example a heat pump, solar thermal system, and wood-fuelled stove with a back boiler.
• House an immersion heater, which could be powered by solar PV panels, a wind turbine or a hydroelectric turbine using a ‘diverter switch’. The immersion heater heats the water in the thermal store.

Thermal stores can vary in size from 250 litres up to 500 litres or more.

You can read more about thermal stores and how they can be integrated with different renewable energy systems.

Heat batteries store either spare heat or electricity, often generated by renewable energy systems. Heat can be stored in a material when it changes phase from a solid to a liquid. These materials are called phase change materials’ (PCM). Spare heat or electricity is used to charge the PCM inside the heat battery. When the heat is needed, the phase change material changes back into a solid with a release of heat, which is used to provide hot water.

Heat batteries are generally smaller and lighter than filled thermal stores. This means you might be able to install one in a convenient location even if you can’t find space for a traditional hot water cylinder. Heat storage batteries do not degrade in the same way as electrical batteries and should have a far longer lifespan.

Thermal stores and thermal batteries are not specifically designed to save money, it’s better to think of them as products that will solve problems or allow other technologies to combine or work more efficiently together. For example, you cannot have a solar thermal system without either a hot water cylinder or a thermal store, but few people would describe a hot water cylinder as specifically saving money. If a thermal store was used to combine heat from a heat pump and wood-fuelled stove with back boiler, it won’t change the amount of energy being produced but it will allow the stove to put energy into a common store and reduce the energy required to be supplied by the heat pump.

A plumber or heating engineer should be able to install a heat battery or thermal store, following the manufacturer’s guidance on correct sizing.

For all energy storage systems, talk to multiple installers for different approaches, system options and quotes. When you’ve decided what system to go for, we recommend getting at least three written quotes to make sure you’re getting the best value for money.

Thermal stores usually do not require notification to building control to install them.