A battery energy storage system (BESS) stores energy and releases it as electricity when needed. Unlike fuel, it doesn’t burn anything — it simply moves energy in and out of rechargeable batteries (storage devices) through chemical reactions. This makes BESS reusable, clean, and a key tool for everything from powering phones to balancing the electric grid.

The batteries discharge to release energy when necessary, such as during peak demands, or grid balancing. In addition to the batteries, BESS requires additional components that allow the system to be connected to an electrical network.

A power conversion system (PCS) is the main device that converts the battery's DC power into AC power for the grid, and vice versa. This allows for power to flow both ways to charge and discharge the battery. The other primary element of a BESS is an energy management system (EMS) to coordinate the control and operation of all components in the system.

For a battery energy storage system to be intelligently designed, both power in megawatt (MW) or kilowatt (kW) and energy in megawatt-hour (MWh) or kilowatt-hour (kWh) ratings need to be specified.

Example:
If a Battery Energy Storage System is rated at 100 MW / 400 MWh, it means:

• Power (MW): The system can discharge up to 100 megawatts at once, this is the “strength” or maximum output at any given moment.

• Energy (MWh): The system can provide that output for 4 hours, since 100 MW × 4 hours = 400 MWh total storage.

So if the same system were instead sized at 100 MW / 200 MWh, it could still discharge 100 MW, but only for 2 hours before the stored energy runs out.

Battery - stores and discharges electrical energy in low voltage battery cells arranged in racks

Storage Enclosure - the physical cabinet or container that holds the battery racks and remaining BESS components

Battery Management System (BMS) – software that ensures the battery cells operate within the correct charging and discharging parameters. The BMS monitors and controls the battery cells’ current, voltage, temperature, state of charge (SoC) and State-of-Health (SoH) to prevent safety risks and ensure reliable operation and performance.

Inverter or a Power Conversion System (PCS) – device that converts the direct current (DC) energy that the battery cell produces into alternating current (AC) used for the power grid, commercial or industrial applications. Bidirectional inverters allow for the charging and discharging of the battery cell.

Energy Management System (EMS) – software that controls and monitors the energy flow of the BESS and systems based on grid signals, market prices or site needs. The EMS coordinates the BMS, inverters and other components of the battery energy system by collecting and analyzing data used to manage and optimize the overall system performance.

Safety Systems – subject to system functionality and operating conditions, a BESS will include fire suppression, smoke detection, a temperature control system, and cooling, heating, and air conditioning systems. A dedicated monitoring and control system will ensure the safe operation of the BESS and the prevention of fire and hazardous incidents. The BESS will also be housed within a secure restricted access area and include CCTV monitoring.

Balancing Supply and Demand
Electricity use goes up and down throughout the day, peaking in the evenings when people get home, cook, and plug in devices. Energy storage charges when demand is low (or when supply is high) and discharges when demand is high, keeping the grid balanced.

Supporting Renewable Energy
Wind and solar don’t always produce electricity at the same time people need it. Storage captures this clean energy and makes it available later, reducing reliance on gas plants and improving grid reliability.

Improving the Grid's Supply Capability
After more than a decade of strong supply, Ontario is entering a period of emerging electricity system needs, driven by increasing demand, the refurbishment of nuclear generating units, as well as end-of-life for many existing power plants. In the grid of the future, BESS will act like a Swiss Army knife, capable of solving many complex electricity supply challenges that our grid currently faces.

Cost Savings for Everyone
Traditionally, Ontario has relied on expensive “peaker plants,” gas plants that only run during the highest-demand hours. Storage can replace much of that role, which keeps costs down. It also delays the need for costly upgrades to transmission lines. Over time, this helps stabilize electricity bills for households and businesses.

Future-Proofing Ontario’s Grid
With nuclear refurbishments, electrification of vehicles and industries, and rising population, Ontario’s electricity needs are changing quickly. Storage ensures we have the flexibility to meet those needs into the 2030s and beyond. So instead of building new power plants, storage ensures supply is available when it’s needed