A modular commercial and industrial (C&I) battery energy storage system (BESS) may seem simple to scale: need more capacity, add another cabinet. In practice, the right cabinet count depends on more than kWh.
One cabinet may be enough if it meets the required power, usable energy, and discharge duration. Multiple cabinets may be needed for higher energy, higher power, longer runtime, or future growth.
How the system scales also depends on the power conversion system (PCS) architecture, charging requirements, transformer capacity, and grid connection. This guide explains how to size a modular C&I BESS for current needs and future expansion.
What Does Scaling a Modular C&I BESS Actually Mean?
In a C&I BESS project, scaling can involve three areas: energy capacity, power, and the wider system. These are related, but they do not necessarily increase in the same way as more cabinets are added.
The result also depends on the BESS architecture. With a battery-only cabinet and shared or centralized PCS, adding cabinets mainly increases battery energy unless PCS capacity is also expanded. With all-in-one cabinets that include their own PCS, adding cabinets can increase both installed energy and rated power.
Energy Capacity Scaling
Energy scaling means increasing how much energy the BESS can store. With identical battery cabinets, installed energy generally increases with cabinet count. Additional energy capacity may support:
- longer discharge or backup duration
- more solar energy shifting
- greater energy availability for daily cycling
The main objective is more kWh, rather than necessarily higher instantaneous output.
Power Scaling
Power scaling means increasing how much power the BESS can charge or discharge at one time. It becomes relevant when a site needs to support higher peak loads, EV charging, or more simultaneous equipment demand.
Whether additional cabinets also increase kW depends on the PCS architecture and rated power.
System-Level Scaling
As a BESS grows, expansion may also affect equipment beyond the battery cabinets, including:
- PCS and transformer capacity
- switchgear, cables, and protection equipment
- energy management system (EMS) and communication architecture
- grid interconnection capacity
A modular cabinet design can simplify expansion, but the wider electrical and control system must also support the intended scale.
One Cabinet or Multiple Cabinets: Which Does Your Project Need?
The number of BESS cabinets should not be based on kWh alone. Start with three project requirements:
- Power (kW): How much power must the BESS deliver at one time?
- Energy (kWh): How much energy must the system store?
- Duration: How long must the required power be sustained?
A useful starting point is: Storage duration ≈ usable energy ÷ discharge power
These values need to work together. A system may have enough energy but not enough power for the load, or enough power but too little usable energy to meet the required duration.
When One Cabinet May Be Enough
One cabinet may be sufficient when its usable energy, available power, and discharge duration all meet the project requirements.
For example, if one cabinet can support the required peak-shaving load for the full target period, adding another cabinet simply to increase installed kWh may offer little practical value.
Future growth should still be considered. Planned PV expansion, EV charging, or new production loads may justify leaving room for additional capacity later.
When Multiple Cabinets Make More Sense
Multiple cabinets become more practical when one cabinet can no longer meet the project's energy or operating requirements.
If energy is the limiting factor, additional cabinets can support:
- longer discharge or backup duration
- more solar energy shifting
- more usable energy for daily cycling
If power is the limiting factor, the expansion path depends on the PCS architecture. A shared or centralized PCS may require additional PCS capacity, while all-in-one cabinets with their own PCS can add both energy and rated power. In either case, the supporting electrical infrastructure must support the higher output.
Future expansion is another reason to consider a multi-cabinet design. A site that fits within one cabinet today may need more energy and power as loads grow.
Cabinet quantity should follow the project's power, energy, duration, and growth requirements-not the other way around.
What Actually Changes When You Add More BESS Cabinets?
Once the project's scaling requirement is clear, the next question is what actually changes when another cabinet is added.
Installed battery energy scales most directly with cabinet count. Power, charging capability, grid-side capacity, and control requirements are more dependent on the BESS architecture and the wider electrical system.
Battery Energy Scales Most Directly
With identical 261 kWh cabinets, installed battery energy scales directly with cabinet count:
- 1 × 261 kWh = 261 kWh
- 2 × 261 kWh = 522 kWh
- 4 × 261 kWh = 1,044 kWh
This additional capacity can support longer discharge periods, more solar energy shifting, or longer backup operation. These figures represent installed energy rather than usable energy. Actual usable energy may be lower because of the SOC operating window, reserve SOC, battery degradation, and temperature-related derating.
System Power Depends on the PCS Architecture
Power does not follow cabinet count in exactly the same way as battery energy.
With a shared or centralized PCS, adding battery cabinets can increase installed energy without increasing PCS rated power. With all-in-one cabinets that each include their own PCS, adding cabinets can increase both installed energy and rated power.
In either architecture, the usable system output may still be limited by:
- PCS quantity and rated power
- Battery current limits and C-rate
- AC bus capacity
- Transformer and switchgear capacity
- EMS operating limits and control strategy
- Grid interconnection limits
The wider electrical system therefore determines how much of the installed power capability can actually be used.
More Energy Can Also Change the Charging Requirement
Adding more cabinets increases the amount of energy that may need to be recharged after discharge.
If charging power remains unchanged, a larger battery system will generally need a longer charging window. Keeping the same recharge time may require more charging power, subject to:
- PCS charging power
- battery charge-current limits
- transformer capacity
- available grid power
Grid-Side Capacity Does Not Scale Automatically
Adding more battery and PCS capacity does not automatically increase the power that the site can import, export, or distribute.
Higher system power may also require sufficient capacity in the:
- transformer
- switchgear and breakers
- busbars
- cables
- grid interconnection capacity at the PCC
Even if the battery and PCS support higher output, existing electrical infrastructure or the permitted import/export capacity at the PCC may limit the power available to the expanded system.
More Cabinets Require More System Coordination
A multi-cabinet BESS must operate as one coordinated system. As cabinet count increases, the control architecture must manage:
- SOC differences between cabinets
- charge and discharge power allocation
- PCS dispatch
- operating limits and status exchange between the battery management system (BMS) and EMS
- alarms and system monitoring
Control and communication capability should therefore be considered part of the expansion design from the beginning.
Quick Summary: What Scales Linearly-and What Doesn't?
The table below summarizes how key system parameters scale as cabinets are added.
| System Parameter | How It Scales | What Else Determines It |
| Installed Battery Energy | Linear with identical cabinet count | Cabinet quantity and rated energy per cabinet |
| Usable Energy | Increases with installed energy, but may be lower than nameplate capacity | SOC window, reserve SOC, degradation, temperature-related derating |
| System Power | Architecture-dependent; may scale with cabinet count when each cabinet includes its own PCS | PCS architecture and rated power, battery current limits and C-rate, AC infrastructure, EMS limits, grid interconnection |
| Charging Capability | Not necessarily linear | PCS charging power, battery charge-current limits, transformer capacity, available grid power |
| Grid Import/Export Capability | May remain unchanged | Transformer, switchgear, cables, permitted import/export power at the PCC |
| System Coordination | Becomes more complex as cabinet count increases | EMS and BMS communication,SOC coordination, powerallocation, PCS dispatch |
Three Common C&I BESS Scaling Paths: Energy, Power, and Staged Expansion
Projects starting with the same 125 kW / 261 kWh C&I BESS can require very different expansion paths depending on whether the constraint is energy, power, or future growth.
Scenario 1: Energy-Led Expansion - More kWh, Similar kW
Consider a site where the 125 kW operating requirement remains sufficient, but the BESS needs to support the load for longer.
Here, the constraint is energy capacity rather than power.
Adding a second identical cabinet increases installed battery energy from 261 kWh to 522 kWh. If the site continues to operate at roughly the same power level, the additional capacity is used mainly to extend discharge duration rather than increase instantaneous output.
Expansion path: 125 kW / 261 kWh → more installed energy with a similar operating power requirement
This is a straightforward energy-led expansion: more kWh without a corresponding increase in required kW.
Scenario 2: Power-Led Expansion - The Load Has Increased
If the additional 261 kWh cabinet is connected to a shared PCS, installed battery energy may increase to 522 kWh while system power remains at 125 kW unless PCS capacity is also expanded.
If each 261 kWh cabinet is an all-in-one unit with its own 125 kW PCS, two cabinets can provide up to 250 kW of installed rated power. However, achieving that output at the site still requires sufficient:
- transformer and distribution capacity
- switchgear, busbars, and cables
- EMS and system control capability
- site and grid interconnection capacity
A power-led expansion therefore requires the battery, PCS, and wider electrical system to be evaluated together.
Scenario 3: Staged Expansion - Size for Today, Design for Tomorrow
A project may not need additional energy or power today. One 261 kWh cabinet may meet current requirements, while future plans include more PV, a larger EV fleet, facility expansion, or higher production loads.
The battery system could then expand in stages:
- Phase 1: 1 cabinet = 261 kWh
- Phase 2: 2 cabinets = 522 kWh
- Future: 4 cabinets = 1,044 kWh
These figures show installed battery energy only-not guaranteed system power. Staged expansion, however, should be planned from the beginning. If future growth is likely, the initial design should consider whether additional cabinets, PCS capacity, electrical infrastructure, and control systems can be added without major redesign.
How to Plan a Modular C&I BESS for Future Expansion
Future expansion should be built into the initial system design. The key is to leave enough flexibility in the site layout, electrical infrastructure, and control architecture to add capacity later without major redesign.
1. Reserve Space for Additional Cabinets
Future cabinets need more than an empty footprint. The initial site layout should also account for:
- additional cabinet positions
- service and maintenance clearance
- future cable routes
- required separation distances
Reserving this space early can reduce civil and installation work when the system expands. It also helps avoid rework or relocation of existing equipment when additional cabinets are added.
2. Plan the PCS and Electrical Design for Expansion
Future cabinets may increase energy capacity, power requirements, or both. The first-phase electrical design should therefore consider whether later expansion can be accommodated without replacing major equipment.
Check the expansion capability of:
- PCS capacity or parallel PCS configuration
- switchgear and breakers
- busbars
- cables
The question is not only whether another cabinet can be installed, but whether the electrical system can integrate it at the intended operating power.
3. Check Transformer and Grid Capacity
Expansion may eventually reach limits outside the BESS itself.
The expected expansion path should be checked against:
- transformer rating
- available grid capacity
- permitted import/export power at the PCC
This matters most when future expansion is expected to increase kW. More battery energy can extend discharge duration without changing site power, while higher output may require upgrades to the transformer or grid connection.
4. Make Sure the Control System Can Scale
Adding cabinets also increases the number of devices, operating limits, and data points that must be coordinated.
The EMS and communication architecture should support:
- the planned number of cabinets
- multi-unit dispatch
- SOC coordination
- power allocation
- communication between the BMS, PCS, and EMS
The control architecture should therefore be checked against the planned system size, not only the first-phase configuration.
5. Confirm Future Battery Compatibility
A cabinet added several years later may not be identical to the original equipment. Product revisions, firmware changes, and battery aging can all affect whether old and new cabinets can operate together.
Future expansion should therefore consider:
- battery chemistry and voltage
- cabinet and module specifications
- BMS and firmware compatibility
- communication protocols
- differences in battery age and state of health
Physical modularity alone does not guarantee that new cabinets can be integrated with an older system without additional engineering checks. Compatibility should be confirmed with the manufacturer before specifying future expansion.
In summary, the right C&I BESS expansion path depends on whether the project needs more energy, more power, or both-and whether the wider system can support it.
Planning a C&I BESS project or future expansion? Share your kW, kWh, operating duration, and expansion plan with Polinovel for a project-specific system configuration.