Batteries store electrical energy. Coupled with renewable generation or an electricity tariff with variable rates, batteries can also help save money.

Storage can also improve supply resilience or be used to reduce the carbon intensity of the electricity that is consumed by a building.

Requires: Photovoltaics, Wind Turbines, Hydroelectric, or suitable electricity tariff.

Batteries store electricity so that it can be used at an alternative time. For example, it may be desirable to store surplus electricity generated by a Photovoltaics system on a weekend for use during the week, or electricity from the grid imported cheaply overnight for the next morning. A battery system could also be used to reduce the carbon intensity of the grid electricity used by a building by storing energy when the grid carbon intensity is low to be used when the grid carbon intensity is higher.

All batteries' performance degrades over time. The lifespan of the battery is dependent on the type of battery, how many charge-discharge cycles it has been through, the charging and discharging routine (has it been charged fully then emptied fully, partially charged then partially emptied, etc.), and the temperature the battery cells have been maintained at. Where active temperature management systems are included with the battery system, preventing excessive temperature variation between day and night, and the seasons, the useful life of the system will be extended.

Battery lifespan is important because current technologies may need to be replaced during the lifespan of a photovoltaics, wind turbine, or hydroelectric system.. Typical battery life may also be shorter than that of the required inverters.

Batteries can either store electricity directly from the generating technology before the inverter (direct current coupling) or after it has been passed through an inverter (alternating current coupling). Inverters are usually installed at the same time as the generating technology so direct current coupled is more suitable if the battery and generating technology are being installed simultaneously, while alternating current coupled is more appropriate when retrofitting a battery to a building with existing generation.

An important consideration when deciding on a battery storage system is whether its only purpose is to store energy for supplementary use later, or whether the battery will also be used as back-up power supply, storing enough electricity to power essential building demands during a power cut. This second requirement is more demanding and will necessitate a larger battery. It will probably also require an alternating current coupling because most direct current coupled systems are not compatible with the provision of back-up power.

Another consideration is any seasonal variation to how the battery will be used. For instance, if a battery is installed for storing excess photovoltaic generation there is likely to be little generation over the winter and no excess electricity needing to be stored. Some battery storage systems are designed for this use case and have a 'winter mode', which reduces the performance degradation to the battery caused by it being near empty for a season.

Some battery storage systems are designed to be ‘stackable’. This makes it easier to install the initial battery storage system, and to increase the capacity in the future. It also reduces the requirement to change the inverter in the future. If you are considering this type of system it is advisable to size the inverter for the maximum anticipated charge/discharge rate in the future, at the point of initial installation.

Batteries are also available with monitoring systems of varying sophistication and compatibility with a Building Energy Management System. This is important because monitoring allows battery storage to be used more effectively, balancing the benefit gained from using the battery with the degradation that inevitably occurs from utilising it. Building energy management systems can enable compatible battery storage systems to import from and export to the grid in a 'smarter' fashion based on previous use patterns or estimated future renewables generation due to weather forecasts, for example.

When installing a battery storage system, the relevant distribution network operator will need to be notified, as may Ofgem (the Office for Gas and Electricity Markets) and the relevant local authority, depending on the size of the storage device and local procedures.

The two most popular kinds of batteries are lead acid and lithium ion, considered in more detail below.26^, Nevertheless there are many other kinds of batteries, such as nickel cadmium, high temperature sodium based, and flow batteries.

Lead Acid

Lead acid batteries are a well-established technology that has traditionally been used with large storage capacities.

An important consideration with lead acid batteries is that over time their performance will degrade quicker if a good charging and discharging routine is not kept.

Lead acid batteries can be vented, valve regulated or thin plate pure lead. Configuration choice will depend on factors such as a building’s electricity demand and generation profiles and its electricity tariff, as well as the required capacity and lifespan, weighed against the extra cost and space required.

Lead acid batteries require good ventilation. It is safest to install them outdoors, but they can also be installed in a well-ventilated, ‘firebox’ room.

Lithium-Ion

Lithium-ion batteries are a newer, more expensive technology than lead acid, however they last longer before needing replacing, and have become widely used over the last ten years.

Compared to lead acid batteries, lithium-ion batteries also:

• Have a higher charge-discharge efficiency, which means less electricity put into the battery when charging is lost in the time before discharging. This makes lithium-ion batteries cheaper to run, because less electricity needs importing from the grid to make up for losses in the battery.
• Are lighter for the same usable storage capacity (due to higher specific energy), which saves on installation costs if the battery needs supporting.
• Are smaller for the same usable storage capacity (due to higher depth of discharge and greater energy density), which saves space.
• Need an integrated controller to manage charge and discharge, unlike lead acid batteries. This is supplied as standard with lithium-ion systems.
• Have greater thermal instability. This means it is strongly recommended they are installed outdoors in specially modified containers.

Lithium-ion batteries can be made of a wide variety of material compositions including iron phosphate, cobalt oxide, nickel cobalt aluminium oxide, nickel manganese cobalt oxide, polymer, titanate, air, and sulphide. Again, the optimal choice will be determined by a building’s electricity profiles and tariff, desired capacity and lifespan, cost, and space required.