What is a battery energy storage system?
Why the grid needs batteries, what a BESS looks like on site, and the handful of terms you need to follow the rest of the course.
A battery energy storage system (BESS) stores electricity when there is plenty of it and gives it back when it is needed. At grid scale, that means rows of shipping-container-sized units full of batteries, connected to the power grid through inverters and a transformer.
Why the grid needs storage
Electricity has always had one awkward property: supply and demand must match every second. For a century, power plants did this by burning more or less fuel. Wind and solar change that. They produce when the weather allows, not when people need power.
On a sunny spring day in Germany, solar panels can produce more than the country uses at midday, and prices can drop close to zero or even below. A few hours later, the sun sets just as people come home, cook and switch on the lights. Demand peaks while solar output falls away.
A battery fills that gap. It charges at midday when power is cheap and plentiful, and discharges in the evening when it is scarce and expensive.
What a BESS looks like on site
A utility-scale BESS is mostly standard equipment arranged in a fenced yard:
- Battery containers. Each holds racks of battery modules, plus cooling and fire safety systems. A modern 20-foot container holds roughly 4 to 5 MWh.
- Inverters (PCS). Batteries store direct current (DC). The grid runs on alternating current (AC). The power conversion system converts between the two.
- Transformers. These raise the voltage so the plant can connect to the grid.
- Controls. An energy management system (EMS) decides when to charge and discharge. A power plant controller makes the whole site behave like one plant towards the grid operator.
Power and energy: the two numbers that describe every BESS
Every project is described with two numbers, for example 50 MW / 100 MWh.
| Number | Unit | What it tells you |
|---|---|---|
| Power | MW (megawatts) | How fast the battery can charge or discharge |
| Energy | MWh (megawatt-hours) | How much electricity it can store |
Divide energy by power and you get the duration: 100 MWh ÷ 50 MW = 2 hours at full power. Most new grid batteries in Europe are built for 1 to 4 hours. Module 2 goes into this in detail.
Which batteries are used
Almost all new grid-scale systems use lithium iron phosphate (LFP) cells. Compared with the chemistries common in electric cars, LFP stores a little less energy per kilogram, but it is cheaper, lasts for more charge cycles and is less prone to overheating. Weight matters far less in a stationary container than in a car.
How a BESS earns money
A battery owner has three broad ways to make money:
- Energy trading. Buy electricity when it is cheap, sell it when it is expensive.
- Grid services. Get paid by the grid operator to stand ready and keep the grid frequency stable.
- Capacity and contracts. Get paid for being available at times of high demand, or rent the battery’s capacity to someone else under a long-term agreement.
Most projects combine several of these. Module 4 explains each one.
Key terms from this module
- BESS: a battery energy storage system connected to the grid.
- MW / MWh: power (how fast) and energy (how much).
- PCS: the inverter that converts between battery DC and grid AC.
- EMS: the software that decides when the battery charges and discharges.
- LFP: lithium iron phosphate, the standard cell chemistry for grid storage.
Next: Module 2 explains the difference between MW and MWh, and why getting it right matters for every other topic.