How to Size a Commercial Battery Energy Storage System

COMMERCIAL BESS SIZING GUIDE

Correct commercial BESS sizing starts with the actual electricity demand of the facility, not simply with choosing a battery capacity from a product list. A system that is too small may not provide enough power or operating time, while an oversized system can add unnecessary project cost.

A practical commercial battery sizing process should determine two values separately: the required power in kW and the required energy capacity in kWh. The final system configuration should also consider backup duration, peak demand, usable depth of discharge, system efficiency, charging conditions, solar PV generation, temperature and future expansion.

If you are new to commercial energy storage, first read our guide on what a commercial energy storage system is and how it works before moving into detailed capacity and power calculations.

1. Define the Purpose of the Commercial BESS

Before starting any BESS capacity calculation, define exactly what the energy storage system is expected to do.

2. Start With the Facility Load Profile

The facility load profile is one of the most important inputs for accurate commercial battery sizing. It shows how electricity demand changes throughout the operating day instead of providing only a single monthly or daily consumption number.

Depending on the project, load data may be recorded every 15 minutes, 30 minutes or one hour. Useful information includes:

• Average operating load
• Maximum demand
• Daily electricity consumption
• Peak-demand periods
• Operating hours
• Critical loads
• Motor, pump and compressor starting loads
• Weekday and weekend demand differences
• Seasonal load variations

For example, a factory may normally operate at 70kW but reach 120kW when several machines start at the same time. A hotel may experience its highest demand in the evening. These facilities can require different BESS configurations even if their total daily electricity consumption is similar.

3. Understand kW vs kWh in BESS Sizing

One of the most common mistakes in kW kWh sizing is treating power and energy as the same specification.

This is why a commercial BESS should not be selected only by comparing battery capacities. Power demand and energy duration must be evaluated together.

4. Calculate the Required Usable Battery Energy

For backup applications, a simple starting point for the BESS capacity calculation is:

Consider a commercial facility with:

• Critical load: 90kW
• Required backup time: 2 hours

90kW × 2 hours = 180kWh usable energy

However, this does not automatically mean that a 180kWh nominal battery is sufficient. The next step is to account for the usable depth of discharge and system losses.

5. Adjust Battery Capacity for DoD and Efficiency

A battery energy storage system normally operates within a defined usable state-of-charge range. The full nominal battery capacity is therefore not necessarily used during every operating cycle.

For a preliminary calculation:

180 ÷ 0.90 ÷ 0.95 ≈ 211kWh

The project would therefore begin evaluation in approximately the 210kWh or higher capacity class. This is a preliminary sizing example rather than a final engineering specification. Reserve requirements, temperature, battery aging and actual system specifications still need to be considered.

6. Determine the Required BESS Power in kW

Battery energy capacity alone is not enough. The PCS must also provide sufficient power for the loads that the system is expected to support.

Pay particular attention to:

• Production machinery
• Electric motors
• Pumps
• Compressors
• HVAC systems
• Refrigeration equipment
• Elevators
• Other high-starting-current loads

A site with a 70kW average load may still require a 100kW or higher PCS if short-duration peak demand must be supported.

7. Compare Commercial BESS Configurations by Power and Energy

Once the project power and energy requirements are known, the next step is to compare actual BESS configurations. The following Voltertech systems illustrate how different combinations of kWh and kW can address different commercial and industrial project requirements.

Quick Comparison

These examples show why product selection should follow the BESS sizing process rather than come before it. A project that requires longer operating duration may prioritize additional kWh, while a project with higher instantaneous load may need additional PCS power.

You can also browse the complete Voltertech Commercial Energy Storage System range for additional capacity and power configurations.

8. Check Available Charging Power

A larger battery also requires sufficient charging capability. If the available charging power is limited, the battery may not fully recharge before the next operating cycle.

Review:

• Grid connection capacity
• Transformer capacity
• PCS charging power
• Available charging window
• Solar PV output
• Generator capacity if applicable

Battery capacity, PCS power and site electrical infrastructure should therefore be evaluated together.

9. Coordinate Battery Capacity With Solar PV

For solar-plus-storage projects, the battery size should be coordinated with actual solar production and facility consumption.

For example, if a commercial facility produces 500kWh of solar electricity during the day but consumes approximately 450kWh during the same period, only part of the remaining solar production is available for charging the battery after system conditions and losses are considered.

Installing additional battery capacity does not create additional charging energy. The PV system, load profile and battery charging strategy must therefore be evaluated together.

10. Consider Temperature, Degradation and Future Expansion

Information Needed Before Final BESS Sizing

For a more accurate system recommendation, prepare the following information before requesting a commercial BESS quotation:

Common Commercial BESS Sizing Mistakes

Sizing Only From Daily Electricity Consumption

Daily electricity consumption does not show when electricity is used or how high the peak load becomes.

Choosing kWh Without Checking kW

A battery may contain enough energy but still be unable to support the required load if PCS power is insufficient.

Ignoring Motor Starting Loads

Pumps, compressors and industrial motors can require substantially more power during startup than during normal operation.

Ignoring the Charging Window

The grid, solar system, generator and PCS must be able to recharge the battery within the available operating schedule.

Oversizing Without a Clear Objective

Additional battery capacity is useful only when it serves a defined operational requirement such as backup, peak shaving, solar utilization or generator reduction.

Frequently Asked Questions

How do you calculate commercial BESS capacity?

A basic calculation starts with load in kW multiplied by the required operating time in hours. The resulting usable energy requirement should then be adjusted for usable depth of discharge, system efficiency, reserve capacity and actual site conditions.

How many kWh does a commercial battery need?

There is no standard capacity for every commercial project. Required battery energy depends on load, required operating duration, peak-shaving strategy, solar production and the main project objective.

Is a 200kWh battery enough for a commercial building?

It depends on the connected load and required operating time. A 200kWh battery supports a 30kW load differently from a 100kW load, so power and energy must be evaluated together.

Should a BESS be sized using average load or peak load?

Both are important. Energy capacity is related to how much energy the system must supply over time, while PCS power must account for the maximum load the BESS is expected to support.

Can a commercial BESS be expanded later?

Some commercial energy storage architectures support modular or parallel expansion. Expansion should be considered during the initial design so the battery architecture, PCS, BMS, EMS and electrical infrastructure remain compatible.

Size the BESS Around the Project, Not Just the Battery

Effective commercial BESS sizing starts with real facility data. The load profile, peak demand, required backup duration, usable battery capacity, PCS power, charging source, solar production, installation conditions and future expansion should be evaluated together.

A 215kWh, 233kWh or 261kWh system should therefore not be selected simply because one capacity appears closest to a calculation result. The final configuration should match both the site's required kW and kWh as well as the wider electrical and operating conditions.

For installers, distributors, EPC contractors and project developers, Voltertech provides commercial and industrial battery energy storage systems for different project power, capacity and application requirements.