In solar power plants, production peaks around midday, while demand and electricity prices often rise in the evening. The way to close that gap is to store energy and use it when it is needed. For ground-mounted plants and large commercial sites, the most common solution is the containerised battery energy storage system, known in the industry as BESS (Battery Energy Storage System) and also referred to as an energy storage container or container battery.
This guide covers the components of a containerised BESS, how capacity is expressed and calculated, what to consider when placing containers on site, and the information that should be stated clearly in the proposal.
Components of a containerised BESS
A containerised BESS is a storage unit built to standard shipping container dimensions and largely prepared in the factory. A typical system includes:
| Component | Function |
|---|---|
| Battery modules and racks | Store the energy; large-scale systems mostly use lithium iron phosphate (LFP) cells |
| Battery management system (BMS) | Monitors cell voltage, temperature and state of charge, keeping the battery within its safe operating range |
| Power conversion system (PCS) | A bidirectional inverter that converts the battery's DC to grid AC and back |
| Energy management system (EMS) | Control software that decides when to charge and when to discharge |
| Thermal management | Keeps cell temperature within range using air conditioning or liquid cooling |
| Fire detection and suppression | Gas, smoke and temperature detection together with a suppression system |
| Transformer and protection | Adapts the voltage level for systems connected at medium voltage |
Some manufacturers supply the PCS and transformer in a separate container or on a skid. It is therefore important to clarify at proposal stage which components "one container" actually includes.
MW and MWh: two different quantities
BESS capacity is expressed with two values, and confusing them is a common mistake:
- Power (MW): the maximum power the system can deliver or absorb at any moment, limited by PCS capacity and the grid connection.
- Energy (MWh): the total amount of energy the system can store.
The ratio of energy to power shows how many hours the system can run at full power. A 2 MW / 4 MWh system, for example, discharges for about 2 hours at full power. The industry also expresses this as the C-rate: 0.5C corresponds to roughly a 2-hour system and 0.25C to roughly a 4-hour system.
How is capacity calculated?
Sizing starts with what the system will be used for. Common applications include:
- Shifting production to the evening: storing midday surplus to use or sell in the evening.
- Reducing curtailment: avoiding the loss of production above the grid connection capacity.
- Peak shaving: lowering the site's maximum demand to reduce capacity charges and grid load.
- Backup power: supplying critical loads during outages.
Once the purpose is defined, the calculation roughly follows these steps (example values):
- Required power: the target is to deliver 2 MW in the evening.
- Required duration: that power is needed for 2 hours, so usable energy is 2 × 2 = 4 MWh.
- Depth of discharge: to protect its life, the battery is not operated across its full nominal capacity. If 90% is usable, nominal capacity is 4 / 0.90 ≈ 4.44 MWh.
- Efficiency and auxiliary consumption: round-trip losses and the consumption of cooling and control systems are added.
- Degradation margin: battery capacity declines over the years. To deliver the target energy over the project life, a margin is built in at the start or additional modules are planned later (augmentation).
- Number of containers: the nominal capacity is divided by the capacity of the selected container.
AC-coupled and DC-coupled systems
A BESS can be connected to a solar plant in two ways:
- AC-coupled: the battery connects to the plant's AC side through its own PCS. This suits retrofits to existing plants and allows independent operation.
- DC-coupled: the battery connects through a DC-DC converter to the DC side of the same inverter as the panels. This allows production above the inverter's AC limit to be stored; the design must be done together with the plant.
The right choice depends on whether the plant is new or existing, the connection capacity and the intended use.
Site layout
Where containers are placed affects both safety and cost. The layout plan should consider:
- Spacing between containers: distances specified by the manufacturer and applicable standards must be maintained for fire safety and maintenance access.
- Foundations: containers usually sit on concrete plinths or foundations; ground investigation and bearing capacity are taken into account.
- Access roads: space is left for delivery, crane unloading and fire service access.
- Cable routes: proximity to the PCS, transformer and connection point determines cable cost and losses.
- Relationship to panels: containers must not shade panel rows or disrupt the overall plant layout.
- Environmental factors: flood risk, high temperatures and noise from cooling systems are considered when choosing the location.
Internationally, BESS safety and performance are addressed by standards such as the IEC 62933 series. Determine the requirements that apply to your project together with an authorised engineer and the relevant authorities.
What the proposal should state clearly
- Power and energy separately: MW and MWh values, with nominal and usable capacity for energy.
- Number of containers and scope: what each container includes, and whether the PCS, transformer and EMS are included.
- Efficiency and auxiliary consumption: round-trip efficiency and the consumption of auxiliary systems such as cooling.
- Warranty: years, cycle count and guaranteed remaining capacity.
- Safety systems: fire detection and suppression, monitoring.
- Operation and maintenance: maintenance scope, remote monitoring and the degradation margin or augmentation plan.
- Regulatory assumptions: connection and licensing conditions for storage depend on current regulations; state them in the proposal as assumptions, not firm commitments.
Self-consumption and price differences matter when assessing the financial impact of storage; see our self-consumption and net metering article and our on-grid, off-grid and hybrid comparison. For smaller battery systems, see our off-grid sizing guide.
Containerised battery systems in PVAGE
In PVAGE, containerised battery systems are handled within ground-mount solar projects. Containers are placed in the ground-mount (mounting structure) design in the same 3D scene as the panel platforms, and a storage capacity calculation is carried out. Container batteries are added to the proposal and shown on drawing sheets such as the single-line diagram and general layout plan.
The plant and the storage system are thus presented with a single design, a single proposal and a consistent set of drawings. See the mounting structure design page and the automatic drawing sheets page for details.