How to Size a Commercial and Industrial Energy Storage System: Power, Capacity and Runtime Explained

In our previous article, "What Is Commercial & Industrial Energy Storage (C&I ESS)? – A Complete Q&A Guide," GSL ENERGY introduced the fundamentals of C&I energy storage. Once the basic concepts are understood, the next practical question is: How should a commercial and industrial energy storage system be sized? In this article, GSL ENERGY, a professional C&I energy storage system manufacturer, takes a closer look at the key factors and calculation methods involved in system sizing.

Sizing a C&I energy storage system is more complex than simply selecting a power rating and battery capacity. It requires assessing the site's load profile, operational objectives, grid interconnection conditions, on-site renewable energy generation, and required backup power duration.

Getting the system size right is critical. If the system is undersized, it may not reduce demand peaks effectively or support critical loads for the required period. If it is oversized, the project will require a higher initial investment, while part of the battery capacity may remain underutilized for long periods.

This guide helps EPC contractors and project developers estimate the required PCS power, battery capacity, and system duration before requesting a detailed energy storage proposal.

Start with the project objective.

The same facility may require very different battery configurations depending on what the owner expects the system to achieve. Before selecting equipment, define the primary use case and rank any secondary objectives.

Common C&I energy storage objectives include:

• reducing peak demand and demand charges;
• shifting electricity consumption from high-price to low-price periods;
• increasing on-site consumption of solar energy;
• maintaining critical operations during grid outages;
• limiting power drawn from a constrained grid connection;
• supporting EV charging or other high-power loads;
• participating in demand response or other grid services;
• stabilizing a microgrid with intermittent renewable generation.

A battery designed mainly for short demand spikes may need a relatively high PCS rating but limited energy capacity. A system intended to provide several hours of backup power will normally require much more battery capacity, even if its power rating is similar.

Understand the difference between kW and kWh

Power and energy describe two different parts of a battery energy storage system.

Kilowatts (kW) indicate how much power the system can charge or discharge at a particular moment. This rating is normally determined by the facility's peak load, target grid limit, and maximum expected power fluctuation.

Kilowatt-hours (kWh) indicate how much energy the battery can store and deliver over time. This determines how long the system can sustain the required output.

For example, a 125 kW/261 kWh system can deliver up to 125 kW under its rated operating conditions. Dividing 261 kWh by 125 kW gives a nominal duration of approximately 2.1 hours. Actual operating time will be affected by usable depth of discharge, conversion efficiency, operating temperature, reserve settings and system controls.

Both values must therefore be calculated independently.

Information required before sizing a C&I BESS

The quality of the system design depends on the quality of the site data. Ideally, the project team should collect at least 12 months of electricity records and interval data at 15-minute or 30-minute resolution.

• The initial dataset should include:
• monthly electricity consumption;
• maximum demand and the method used by the utility to calculate it;
• weekday and weekend load curves;
• seasonal load variations;
• electricity tariffs and time-of-use periods;
• existing or planned solar PV capacity;
• solar production and export data, where available;
• grid connection capacity;
• critical and non-critical load lists;
• expected backup duration;
• available installation area and environmental conditions;
• planned production lines, chargers or other future loads.

Monthly electricity bills alone rarely show how long a demand peak lasts. Interval data is particularly important for deciding whether a site requires a short-duration, two-hour or four-hour battery system.

Step 1: Calculate the required PCS power

The power conversion system must be able to respond to the site's required charge and discharge power. The appropriate method depends on the application.

PCS power for peak shaving

For a peak-shaving project, a preliminary discharge power requirement can be calculated as:

Required discharge power (kW) = Site peak load − Target grid demand

If a factory reaches 320 kW and wants to keep grid demand below 200 kW:

320 kW − 200 kW = 120 kW

The storage system must therefore be capable of supplying at least 120 kW during the peak. A 125 kW PCS would provide a close technical match, subject to a detailed review of load fluctuations, response time and operating reserve.

PCS power for backup operation

For backup power, add the simultaneous power requirements of all critical loads. Motors, pumps, compressors, and HVAC equipment may draw high starting currents, so their startup characteristics must be reviewed rather than relying only on nameplate running power.

If the critical-load total is 90 kW, the PCS must continuously support at least that load. Additional headroom may be required for startup power, load changes, and future expansion.

PCS power for solar energy management

In solar self-consumption projects, the PCS must absorb the expected PV surplus without exceeding its charge limit. The project team should compare midday export power, battery charging time, and the site's evening demand.

Installing more battery capacity without sufficient charging power may prevent the battery from capturing the available solar surplus within the required time window.

Step 2: Calculate the required battery capacity

Once the power requirement is known, calculate how long the battery must maintain that output.

A basic energy calculation is:

Required delivered energy (kWh) = Required power (kW) × Operating duration (hours)

If the system must supply 120 kW for two hours:

120 kW × 2 hours = 240 kWh

However, 240 kWh is the required delivered energy, not necessarily the nominal battery capacity. A realistic design must account for usable depth of discharge and system efficiency.

Required nominal capacity = Required delivered energy ÷ (Usable depth of discharge × System efficiency)

Using preliminary planning assumptions of 90% usable depth of discharge and 92% system efficiency:

240 kWh ÷ (0.90 × 0.92) = approximately 290 kWh

These percentages are examples for early-stage estimation. Final calculations must use the certified data, warranty conditions, and operating limits of the selected battery and PCS.

Step 3: Choose the appropriate storage duration

Storage duration is the relationship between battery energy capacity and discharge power. The right duration is determined by the length of the business problem the battery needs to solve.

A four-hour system is not automatically better than a two-hour system. The battery should be aligned with the actual load curve and dispatch schedule. If the facility's peak lasts only 45 minutes, a four-hour battery may leave a large share of its capacity unused unless it also performs energy arbitrage, backup, or renewable-energy shifting.

Step 4: Account for usable capacity and operating reserve

The nominal capacity printed on a battery specification is not always the same as the energy available to the load. The system designer should distinguish among:

• nominal battery capacity;
• usable battery capacity;
• energy available after PCS and auxiliary losses;
• emergency reserve maintained by the EMS;
• capacity available at the end of the project period.

For sites requiring backup power, the EMS may maintain a minimum state of charge rather than using the entire battery for daily tariff optimization. For example, reserving 20% of the battery for outages reduces the energy available for peak shaving during normal operation.

The control strategy must therefore be considered during sizing, not added after the equipment has been selected.

Step 5: Consider degradation and future expansion

Battery capacity gradually declines through calendar ageing and cycling. A system sized only for its first year may no longer meet the full runtime requirement later in the project.

• The design review should consider:
• the required capacity at the end of the warranty period;
• expected cycles per day;
• average and maximum depth of discharge;
• operating temperature;
• charging and discharging power;
• the manufacturer's retained-capacity warranty;
• whether future augmentation is technically and commercially practical.

Expansion planning is also important for growing factories, commercial campuses and EV charging sites. A modular system can simplify later capacity additions, but compatibility between old and new battery clusters, PCS limits, switchgear capacity and site layout must be evaluated in advance.

Worked example: sizing a factory peak-shaving system

Consider a factory with the following preliminary data:

• recorded peak demand: 320 kW;
• target grid demand: 200 kW;
• typical peak duration: two hours;
• no backup reserve included in the first calculation;
• preliminary usable depth of discharge: 90%;
• preliminary system efficiency: 92%.

1. Calculate discharge power

320 kW − 200 kW = 120 kW

The PCS should supply at least 120 kW, so a 125 kW system is a logical starting point.

2. Calculate delivered energy

120 kW × 2 hours = 240 kWh

3. Adjust for usable depth and efficiency

240 kWh ÷ (0.90 × 0.92) = approximately 290 kWh nominal capacity

In this simplified scenario, a 261 kWh system may control the full 120 kW peak for less than two hours, or it may meet the objective if the actual load falls during the peak period. A 418 kWh system provides more energy margin and may also support a backup reserve, longer peak control, or future load growth.

The final choice should be validated by simulating the complete interval load profile rather than using only the single maximum-demand value.

125kW/261kWh or 125kW/418kWh: which configuration fits?

GSL ENERGY offers liquid-cooled C&I configurations that can be evaluated against different load durations and operating objectives.

Project developers can review the 125kW/261kWh liquid-cooled energy storage system and the 125kW/418kWh liquid-cooled BESS as reference configurations. Product selection should still be based on project-specific electrical and operating data.

Sizing for solar self-consumption

For a solar-plus-storage project, start with the PV energy that would otherwise be exported or curtailed.

A preliminary daily capacity estimate is:

Required usable battery capacity = Daily surplus solar energy intended for storage

Suppose a commercial site exports approximately 300 kWh between late morning and early afternoon, while evening operations can consume the same amount. A battery with about 300 kWh of usable capacity may be considered before adjusting for conversion losses, reserve settings, and seasonal variations.

The PCS charging power must also be sufficient. If the 300 kWh surplus is available over only two hours, the system may need to absorb an average of 150 kW. A 125 kW PCS would capture only part of that surplus unless the charging window is longer, the export profile is less concentrated, or multiple systems are used.

This illustrates why battery capacity and PCS power must be assessed together.

Sizing for backup power

Backup sizing begins with a critical-load schedule rather than the facility's total connected load.

The basic calculation is:

Required delivered energy = Critical load × Required backup time

If a facility needs to support 80 kW of critical loads for three hours:

80 kW × 3 hours = 240 kWh delivered energy

The nominal battery capacity must then be adjusted for usable depth of discharge, conversion efficiency, reserve margin and ageing. The system must also be able to operate in island mode, establish or follow a stable microgrid and coordinate with generators, solar inverters and transfer equipment where applicable.

A battery that is correctly sized in kWh may still fail as a backup source if the PCS, transfer architecture or motor-starting capability is inadequate.

Common C&I energy storage sizing mistakes

Using monthly consumption instead of interval data

Two facilities with the same monthly electricity use can have very different peak demand patterns. Monthly bills do not provide enough detail for reliable peak-shaving design.

Treating nominal capacity as fully usable energy

Ignoring depth of discharge, conversion losses, and reserve settings can produce an undersized system.

Selecting kWh before calculating kW

A large battery cabinet cannot control a sharp demand spike if its PCS cannot deliver the required power.

Ignoring the charging window

The system must have enough time and available grid or solar power to recharge before the next discharge event.

Sizing only for the first year

Load growth and battery degradation should be incorporated into the long-term design.

Combining every objective without setting priorities

The same stored energy cannot always be committed simultaneously to peak shaving, price arbitrage, and emergency backup. The EMS strategy and reserved capacity must reflect the owner's priorities.

What to provide when requesting a C&I BESS proposal

To obtain a technically meaningful quotation, provide the system supplier with:

1. project location and installation environment;
2. utility voltage, frequency and grid connection capacity;
3. at least several weeks of interval load data, preferably 12 months;
4. electricity tariff and demand-charge structure;
5. existing or planned solar PV and generator capacity;
6. required backup loads and runtime;
7. available installation area;
8. required certifications or local grid standards;
9. preferred operating objectives and their priority;
10. expected future load growth and project schedule.

With this information, a supplier can evaluate the PCS rating, battery capacity, cabinet quantity, control strategy, electrical architecture, and expansion plan.

Frequently asked questions

How do I calculate the size of a commercial battery storage system?

First, calculate the maximum charge or discharge power in kW. Then multiply the required output by the operating duration to estimate delivered energy in kWh. Finally, adjust for usable depth of discharge, efficiency, operating reserve, degradation,n and future expansion.

Is a 100 kW/200 kWh battery a two-hour system?

Its nominal energy-to-power ratio is two hours. The actual duration at 100 kW will normally be shorter after accounting for usable capacity, system losses, reserve settings, and operating conditions.

How much battery storage does a factory need?

There is no standard capacity for every factory. Required storage depends on the load profile, peak-demand target, electricity tariff, solar generation, critical loads, and required runtime.

Should battery capacity match the total daily electricity consumption?

Usually not. Most C&I systems are sized for a defined portion of the load, such as peak periods, surplus solar generation, or critical backup circuits. Matching the battery to total daily consumption can result in substantial oversizing.

Can a C&I energy storage system be expanded later?

Many systems support modular expansion, but the original design must consider PCS capacity, battery compatibility, DC architecture, switchgear, communications, available space, and commissioning requirements.

Is a two-hour or four-hour BESS better for C&I applications?

The better option is the one that matches the site's operating objective. Two-hour systems are often suitable for shorter demand peaks and tariff management, while longer-duration systems may be more appropriate for extended solar shifting, weak-grid operation, or backup power.

Build the system around operating data.

A successful commercial and industrial energy storage system begins with site data, not a standard cabinet size. PCS power must match the instantaneous load requirement, while battery capacity must support the required duration after accounting for usable energy, efficiency, operating reserve, and long-term degradation.

GSL ENERGY provides modular high-voltage batteries, all-in-one liquid-cooled cabinets and containerized BESS solutions for commercial, industrial and multi-megawatt-hour projects. Project teams can submit their load profile, solar configuration, tariff structure, and backup requirements for a project-specific capacity assessment and system proposal.