What is BESS, Battery Energy Storage System?

A Battery Energy Storage System (BESS) refers to a system that uses rechargeable batteries to store energy and release it as needed. The core component of a BESS is the battery itself, which is typically based on lithium-ion technology, although other types like lead-acid, flow batteries, and solid-state batteries are also used in various applications. BESS can vary in size, from small units for individual homes to large, grid-scale solutions for utility companies.

This technology is becoming increasingly important as our energy system shifts toward renewable sources like solar and wind, which don’t always produce energy when demand is highest. With BESS, surplus energy generated during periods of low demand or high production can be stored and discharged later. This grants both a benefit for the system and a profit for the BESS: excess renewable generation typically depresses prices, meaning the energy injected in the battery is bought at a low price; at the opposite, this same energy is then released when demand and prices are higher.

The very quick response times characterizing BESS also allow them to provide ancillary services to the grid, both contributing to system security and flexibility and generating an additional revenue stream for the BESS.

The main technical parameters used to define BESS are:

• Efficiency – the roundtrip efficiency of a BESS system defines is the ratio between the energy returned to the grid and the energy extracted from it. It is generally lower than 100% due to losses incurred during injection, storage and withdrawal.
• Energy storage capacity – the maximum energy than can be stored in the BESS, usually expressed in kWh or MWh.
• Power – the maximum charge or discharge rate of the BESS, usually expressed in kW or MW
• Duration – the ratio between energy storage capacity and power, indicates how long it takes for the BESS to fully charge or discharge, usually expressed in hours.
• C-rating – an alternative way to indicate how long it takes for the BESS to fully charge or discharge, it is calculated as the inverse of the duration (e.g. 0.5C means 2hr duration and 1C means 1hr duration).