What is a BESS? A plain-English guide for UK businesses

A BESS is a battery energy storage system: a set of batteries, an inverter, control software and protection equipment that together store electricity and release it when it is needed. The term covers everything from a cabinet in a plant room to a field of shipping-container-sized units. This guide is about the commercial and industrial end, the systems a business installs on its own side of the meter to cut its bill, ride out peaks or keep its site running. For what it costs and the rules that apply, see our buying guide to commercial battery storage.

What BESS stands for

BESS stands for battery energy storage system, and you will sometimes see battery electrical storage system. Energy storage system (ESS) is the wider term and includes storage that is not battery-based, such as pumped hydro. UK electrical standards use a third form: the IET's Code of Practice for Electrical Energy Storage Systems is named for the electrical energy storage system, or EESS. The three terms overlap, and in a commercial setting they usually mean a lithium-ion battery system connected to a building's electrical supply.

The parts of a BESS

Every BESS, large or small, is built from the same functional parts:

• Battery cells, modules and racks. The cells hold the energy. They are grouped into modules and racks. For commercial storage the two common lithium-ion chemistries are lithium iron phosphate (LFP) and nickel manganese cobalt (NMC), and the NREL Annual Technology Baseline covers both.
• Battery management system (BMS). Electronics that monitor each cell's voltage and temperature, keep it inside safe limits and report its state of charge.
• Power conversion system (PCS). The inverter. Batteries store direct current, the building uses alternating current, and the PCS converts in both directions. Its rating sets the kW the system can deliver.
• Energy management system (EMS). The controller that decides when to charge and discharge: overnight when power is cheap, through the afternoon peak, or when the grid fails.
• Thermal management and enclosure. Cooling or heating to keep the cells in their working range, inside a cabinet, room or container.
• Protection, switchgear, metering and fire detection. Isolators, breakers, meters and detection, plus suppression where the design calls for it.

The cost of a system follows the same split: NREL's commercial battery model prices the pack per kWh and the power equipment per kW, with a fixed amount on top. Our guide to commercial battery storage cost shows the published UK bands.

How a BESS works

The principle is simple: take in power when it is cheap or plentiful, give it back when it is expensive or scarce. In a business that usually means charging overnight or from the site's own solar panels and discharging through the weekday peak. Some energy is lost each time. NREL's model assumes a round-trip efficiency of 85%, which on a 100 kWh charge returns about 85 kWh. It is a planning assumption, and your supplier should state what theirs is measured against.

Behind the meter and front of the meter

The same technology is used in two very different ways. A behind-the-meter BESS sits on a customer's side of the meter and serves that site, cutting what it draws from the network, absorbing its own solar and sometimes taking part in flexibility markets. A front-of-the-meter or grid-scale BESS connects directly to the network and trades electricity, often at tens or hundreds of megawatts. The commercial guidance on this site is about the first kind.

What a business uses a BESS for

• Peak shaving: discharging at the site's peak to reduce time-banded charges and the capacity it pays for. Our page on DUoS charges explains what those charges are.
• Energy arbitrage: buying power when it is cheap and using it when it is dear.
• Storing solar: using more of the site's own generation instead of exporting it at a low price.
• Backup power: keeping chosen loads running when the grid fails, which needs the battery to hold a reserve.
• Grid services: earning for flexibility through a supplier or an aggregator, where the site qualifies.

How a BESS is sized: kW and kWh

A BESS is specified by two numbers that answer different questions. Power, in kW, is how fast it can charge or discharge. Energy, in kWh, is how much it can hold. Divide one by the other and you have its duration: 250 kWh at 125 kW is a two-hour system. Choosing the wrong one is a common and expensive mistake, and our guide to kW vs kWh explains how to work out which you need.

UK standards and safety

Several documents apply to a commercial BESS, and they cover different things:

• IET Code of Practice for Electrical Energy Storage Systems, 3rd edition (2024). It covers the specification, design, installation, commissioning, operation and maintenance of systems in dwellings, commercial and multi-occupancy buildings and industrial sites, for both AC and DC-coupled low-voltage systems.
• BS 7671:2018+A4:2026. Amendment 4 of the Wiring Regulations was published on 15 April 2026 and adds Chapter 57, Stationary secondary battery installations. The previous version is withdrawn on 15 October 2026.
• IEC 62619:2022. Safety requirements for secondary lithium cells and batteries in industrial applications, including stationary storage.
• IEC 62933-5-2:2025. Safety requirements for grid-integrated electrochemical energy storage systems across the whole life of the system.

You may see PAS 63100 cited as a standard for commercial batteries. It is BSI's specification for fire protection of battery systems in dwellings, so it is not the commercial reference. If a supplier relies on it for a business site, ask which commercial standard they are working to.

Grid connection

A BESS that can export more than 16 A per phase needs permission from the network operator through a G99 application, and smaller export-capable systems follow the simpler G98 route. Our page on the G99 application explains the process and why it can set the programme.

BESS, UPS, generator and solar: what is the difference?

People often confuse these because they all sit in a plant room. A BESS stores energy and moves it in time. A UPS (uninterruptible power supply) is a smaller battery built to bridge a power cut for seconds or minutes so equipment can ride through or shut down safely. A generator makes energy from fuel and takes time to start. Solar panels generate when the sun shines and store nothing. A BESS can do some of a UPS's job and some of a generator's, but only if it is designed for it: a battery sized for peak shaving may not hold enough reserve for backup.