An energy storage system saves electricity so you can use it later. Most of the time that means batteries. You will often see them called a battery energy storage system, or BESS.

The idea is simple. Electricity is not always made at the moment it is needed. Solar panels produce the most in the middle of the day, but many buildings use a lot of power in the evening and overnight. A battery fills that gap.

What is inside a storage system

Batteries. These hold the energy. Most modern systems use lithium-ion batteries.
An inverter and charger. Batteries store direct current (DC), which flows in one direction. Buildings run on alternating current (AC). The inverter converts between the two, both when charging and when supplying power.
A battery management system, or BMS. This is the electronic brain that watches every part of the battery. It keeps it within safe limits for temperature, charge and current.
Controls. These decide when to charge, when to supply power, and when to draw from the grid.

What batteries are used for

Storing solar power. Spare solar energy from the day is saved and used at night.
Backup power. If the mains supply fails, a properly set-up system can keep important equipment running.
Cheaper electricity. Some electricity tariffs are cheaper at night. A battery can charge then and supply the building when power costs more.
Reducing peaks. Large sites can use batteries to cover short bursts of high demand, so they draw less from the grid at once.
Off-grid power. Where there is no grid connection at all, batteries are the heart of the system.

Getting the size right

Two numbers describe a battery system. Capacity, measured in kilowatt hours (kWh), is how much energy it can hold. Power, measured in kilowatts (kW), is how fast it can deliver that energy.

As a minimum, a battery system should be sized to cover night-time use, taking into account what the solar panels produce during the day. Some sites need much more, enough to run a building through several days of dull weather. A building that needs a lot of power at once needs a system with a high kW rating as well as enough kWh.

Battery types

Older systems often used lead-acid batteries, the same basic type used to start a car. They work, but they do not like being drained deeply and recharged every day.

Lithium-ion batteries cope much better with daily use. More of their stored energy can be used without harming them. So a lithium-ion battery can be more useful in practice than a lead-acid battery with a bigger number on the label.

Homes and industrial sites

The principles are the same at any size. A small system and a large industrial one both store energy, convert it and control when it is used. Industrial systems are larger, connect to bigger electrical supplies and often have more complex controls. Our own work is on commercial and industrial sites.

Other ways to store energy

Batteries are not the only option. Much of the energy a building uses goes on heating, so surplus power can also be stored as hot water in a well-insulated tank. These are sometimes called water batteries, and they work well alongside electrical storage.

Safety and lifespan

Modern battery technology is cost-effective and reliable, and it lasts a long time, as long as systems are designed and installed properly. That means the right location, ventilation, protection devices and cabling, and a battery management system that is set up correctly.