How to Size a C&I Battery Energy Storage System Using a Load Profile

Choosing the right size for a commercial and industrial battery energy storage system is not simply a matter of selecting a battery with a large kWh rating.

A site may say that it needs a 200kWh or 300kWh battery, but without understanding the facility’s actual load profile, it is difficult to know whether that energy capacity is appropriate.

The key questions are different.

How high is the facility’s peak demand? How long does that peak last? How much grid demand needs to be reduced? Does the site experience one peak per day or several? Is solar PV already installed? Does the battery also need to reserve energy for backup power?

These questions determine both the required power in kW and the required energy capacity in kWh.

For this reason, a proper C&I BESS sizing process should begin with the site’s load profile rather than with a preferred battery size.

What Is a Load Profile?

A load profile shows how a facility’s electricity demand changes over time.

Instead of looking only at total daily or monthly energy consumption, the load profile shows when electricity is being used and how much power is required at different times of day.

For example, a factory might have the following operating pattern:

Base Load = 120–150kW

Normal Daytime Load = 180–220kW

Peak Load = 320kW

Target Grid Limit = 220kW

The difference between the actual peak and the target grid limit is:

320kW − 220kW = 100kW

This means the battery may need to provide approximately 100kW during the highest part of the peak.

But that still does not tell us how much energy capacity is required.

For that, we also need to know how long the peak lasts.

Power and Energy Solve Different Problems

One of the most important steps in BESS sizing is separating the power requirement from the energy requirement.

Power (kW) determines how much of the site’s demand the battery can support at one moment.

Energy (kWh) determines how long the battery can maintain that level of support.

For peak shaving, a useful preliminary relationship is:

Required Battery Power = Actual Grid Peak − Target Grid Demand

If the current grid peak is 320kW and the target grid limit is 220kW:

320kW − 220kW = 100kW

The preliminary battery power requirement is therefore approximately 100kW.

If that 100kW reduction is required for 1.5 hours:

100kW × 1.5 hours = 150kWh

The preliminary energy requirement becomes approximately 150kWh.

This gives an initial sizing point of roughly:

100kW / 150kWh

However, this is only the theoretical starting point.

Why Peak Duration Matters So Much

Two facilities can have exactly the same maximum demand and still require completely different battery capacities.

Consider two factories that both need to reduce their grid demand by 100kW.

Factory A

The peak lasts approximately 15 minutes.

The theoretical energy requirement is:

100kW × 0.25 hours = 25kWh

Factory B

The peak lasts approximately 3 hours.

The theoretical energy requirement is:

100kW × 3 hours = 300kWh

Both sites require roughly the same 100kW battery power.

But one theoretically needs only about 25kWh of energy for that peak, while the other needs approximately 300kWh.

This leads to one of the most important rules in C&I storage sizing:

Peak magnitude determines the required kW. Peak duration determines the required kWh.

Looking only at the maximum demand is therefore not enough.

Why 15-Minute or 30-Minute Interval Data Is Useful

Monthly electricity bills are useful for understanding overall consumption and costs, but they usually do not provide enough detail for accurate BESS sizing.

For commercial and industrial projects, interval load data is much more useful.

Depending on what the utility meter or energy-management system provides, this may include:

• 15-minute demand data
• 30-minute demand data
• Hourly demand data
• Real-time monitoring data

The shorter the interval, the easier it is to understand the shape and duration of individual peaks.

A 300kW maximum recorded once during a month tells us very little by itself.

The important questions are:

How often does the site reach 300kW?

Does the peak last 15 minutes or three hours?

Does it happen every weekday?

Are there several peaks in one day?

Does the peak occur at the same time as high electricity prices?

A BESS should be sized around the actual operating pattern, not just the single highest number on the bill.

Step 1: Identify the Base Load

The first useful quantity is the facility’s normal or base load.

Suppose a factory typically operates between:

120kW and 160kW

during low-production periods.

This tells us the minimum level of demand that is normally present.

Understanding the base load is important because the battery needs a suitable charging window.

If site demand already stays high throughout the entire day, it may be more difficult to charge aggressively without creating another demand peak.

Step 2: Identify the Maximum Demand

Next, identify the highest grid demand recorded during normal operation.

Assume:

Maximum Demand = 320kW

This is the peak that the system may need to control.

However, the maximum demand alone does not determine BESS size.

The target demand must also be defined.

Step 3: Define the Target Grid Demand

Peak shaving requires a target.

Suppose the facility wants to limit grid demand to:

Target Grid Demand = 220kW

The required reduction becomes:

320kW − 220kW = 100kW

This means the battery and PCS need sufficient discharge capability to provide approximately 100kW when the site reaches its highest demand.

If the required reduction is larger, the required BESS power also increases.

For example:

This is why the target grid limit has a direct influence on PCS and battery power sizing.

Step 4: Measure How Long the Peak Lasts

After determining the required peak reduction, the next step is measuring how long the battery must maintain that power.

Suppose the site’s 100kW reduction is required for:

1.5 hours

The theoretical energy requirement becomes:

100kW × 1.5h = 150kWh

If the same peak lasts for two hours:

100kW × 2h = 200kWh

If it lasts for three hours:

100kW × 3h = 300kWh

This shows why simply saying “the site needs 100kW of peak shaving” is not enough.

The project also needs to answer:

For how long?

Step 5: Check Whether There Is More Than One Peak

Real facilities often experience several peaks during the same day.

For example:

10:00 — 260kW

14:00 — 320kW

18:00 — 280kW

If the battery discharges heavily during the first peak, there may not be enough energy remaining for the second or third peak.

This is where daily SOC trajectory becomes important.

The design should consider:

• Battery SOC before the first peak
• Energy used during each peak
• Time available between peaks
• Whether the battery can recharge between peaks
• Charging-power limits
• Whether solar PV can contribute to recharging

A system that appears large enough for one individual peak may still be undersized for an entire day of repeated peak-shaving events.

Example: Multiple Peaks in One Day

Suppose the battery begins the day at 90% SOC.

The first peak requires 50kWh.

The second peak requires 100kWh.

The third peak requires another 60kWh.

The total theoretical daily discharge requirement is:

50kWh + 100kWh + 60kWh = 210kWh

If there is no opportunity to recharge between these peaks, the system needs enough usable energy to support the full daily requirement.

But if the site has a suitable charging window between the first and second peak, the required installed energy capacity may be different.

This is why BESS sizing is not simply a matter of calculating one peak and multiplying power by time.

The whole operating day needs to be considered.

Step 6: Account for Usable Battery Energy

Nominal battery capacity is not always the same as the amount of energy available for the application.

A BESS may reserve part of its state of charge for battery protection, operating strategy or backup power.

For example, if a system has:

Nominal Capacity = 200kWh

but only 90% of that capacity is intended for the daily operating window, the available energy before other losses would be approximately:

200kWh × 90% = 180kWh

This is why the practical battery capacity normally needs to be larger than the simplest theoretical calculation.

The exact usable-energy window depends on the battery system, operating strategy and project requirements.

Step 7: Consider Conversion Efficiency

Energy passes through several components before reaching the facility.

Battery discharge energy is converted through the PCS, and each conversion introduces some loss.

If the application requires 200kWh to reach the AC side, the battery may need to supply more than 200kWh internally.

The exact adjustment depends on the efficiency assumptions used for the system.

For preliminary sizing, it is important to distinguish between:

Battery-side stored energy

and

AC-side delivered energy

This avoids selecting a system that looks large enough on paper but cannot deliver the required usable energy during real operation.

Step 8: Include an Operating Margin

Real-world load profiles are not perfectly predictable.

Production schedules change.

Equipment is added or removed.

HVAC demand varies with weather.

Peak duration may occasionally be longer than historical averages.

For these reasons, it is generally useful to include an appropriate operating margin when moving from a theoretical calculation to a practical system design.

The goal is not simply to meet one ideal historical day.

The system should be able to operate reliably across the range of conditions expected at the site.

How Solar PV Changes BESS Sizing

If a facility already has solar PV, the BESS should not be sized using the raw facility load alone.

The more useful quantity is the net grid load.

A simplified relationship is:

Net Grid Load = Facility Load − Solar Generation

Suppose:

Facility Load = 300kW

Solar PV Output = 80kW

The grid sees:

300kW − 80kW = 220kW

If the site’s target grid limit is also 220kW, the battery may not need to discharge at that moment.

Now suppose the same facility remains at 300kW later in the afternoon, but PV production falls to 20kW.

Net grid demand becomes:

300kW − 20kW = 280kW

To maintain the 220kW target, the battery would need to supply approximately:

280kW − 220kW = 60kW

This example shows why PV generation should be analyzed together with the load profile.

Facility Load vs. Net Grid Load

For sites with solar, it is useful to keep two curves separate.

Facility Load Curve shows how much electricity the site is actually consuming.

Net Grid Load Curve shows how much electricity the utility grid must supply after solar generation and battery operation are considered.

For peak-shaving projects, the net grid curve is often the more important control target.

The EMS can monitor the grid connection point and adjust battery output so that grid demand stays below the desired threshold.

What If Solar Charges the Battery?

Solar can also affect the energy-capacity calculation.

If excess PV is available during the middle of the day, the battery may recharge before a later afternoon or evening peak.

This can reduce the amount of energy that needs to be stored at the start of the day.

For example, the BESS may discharge during a morning peak, recharge from excess PV around midday, and then discharge again during the late-afternoon peak.

This type of operation can improve battery utilization, but it also makes the sizing analysis more dynamic.

The designer needs to consider not only the amount of energy required, but also when energy enters and leaves the battery throughout the day.

How the Charging Window Affects System Size

Charging is an important part of BESS sizing.

Suppose the battery needs to recover 200kWh before the next peak.

If there are four hours available for charging, the average battery charging power required would theoretically be:

200kWh ÷ 4 hours = 50kW

If only one hour is available:

200kWh ÷ 1 hour = 200kW

The available charging window can therefore influence PCS power requirements and the overall operating strategy.

The battery should also avoid creating a new unwanted grid peak while charging.

A well-designed EMS considers both facility demand and battery charging power at the same time.

Peak Shaving and Backup Power Require Different Reserves

Some C&I projects expect the same BESS to support both peak shaving and emergency backup.

This changes the sizing process.

Suppose a 258kWh battery is installed, but the project requires 80kWh to remain reserved for backup.

That means the entire 258kWh should not be assumed to be available for daily peak shaving.

The EMS may maintain a minimum SOC so that backup energy remains available in case the utility grid fails.

If both functions are required, the battery needs to be sized around the combined operating strategy.

This is one reason a BESS that looks large enough for peak shaving alone may still be too small when backup requirements are added.

Example: Could a 125kW / 258kWh BESS Fit This Load Profile?

Consider the following simplified facility:

Peak Load = 320kW

Target Grid Demand = 220kW

Required Peak Reduction = 100kW

Typical Peak Duration = 1.5 to 2 hours

The required battery power is approximately:

320kW − 220kW = 100kW

A system with a 125kW power capability would theoretically provide enough power to cover this 100kW peak-reduction target.

Now consider the energy requirement.

At 100kW for 1.5 hours:

100kW × 1.5h = 150kWh

At 100kW for two hours:

100kW × 2h = 200kWh

A 258kWh system therefore appears, at a preliminary level, to provide enough nominal energy capacity for this simplified application.

However, this does not mean that a 125kW / 258kWh BESS is automatically the correct system for the project.

The final evaluation still needs to consider usable SOC range, conversion efficiency, reserve requirements, repeated peaks, PV generation, charging windows, battery degradation and EMS strategy.

The example simply demonstrates how a real BESS specification can be compared with a site’s load-profile requirements.

Why Oversizing Is Not Always Better

It may seem safer to select a much larger battery than the calculations suggest.

However, oversizing can increase project cost without necessarily improving the business case.

A battery that is rarely charged or discharged through a meaningful operating range may provide limited additional value.

Similarly, installing much more PCS power than the site’s peak reduction requires may not improve normal operation.

The objective should therefore be right sizing, not simply maximum sizing.

A well-matched system balances:

Battery Power

Battery Energy

Site Load

Solar Generation

Charging Window

Tariff Structure

Backup Requirements

and

Future Expansion

Future Load Growth Should Be Considered

A site’s current load profile may not represent its future load profile.

A factory may add new production equipment.

A warehouse may install EV chargers.

A commercial site may expand its HVAC system.

Additional solar PV may also change the site’s daytime grid demand.

Before finalizing BESS size, it is useful to ask whether major electrical changes are expected over the next several years.

A system that is perfectly sized for today’s load may become undersized after significant site expansion.

A Practical C&I BESS Sizing Workflow

A preliminary BESS sizing process can follow this sequence:

1. Collect the Load Profile

Use 15-minute, 30-minute or the most detailed available interval data.

2. Identify the Current Peak

Determine the site’s actual maximum grid demand.

3. Set the Target Grid Limit

Define how much peak reduction is required.

4. Calculate Required Power

Use the difference between peak demand and the target grid demand.

5. Measure Peak Duration

Determine how long the required reduction must be maintained.

6. Calculate Theoretical Energy

Multiply required power by duration.

7. Analyze Multiple Daily Peaks

Check whether the BESS needs to manage several peaks before recharging.

8. Include Solar PV

Use net grid load where solar generation is present.

9. Consider Efficiency and Usable SOC

Convert the theoretical requirement into a more realistic usable-energy requirement.

10. Check Charging, Backup and Future Expansion

Verify that the battery can recharge in time and preserve any required backup reserve.

This process provides a much stronger starting point than selecting a BESS based only on a preferred kWh value.

What Data Should You Prepare for a BESS Supplier?

Before requesting a C&I storage proposal, it is useful to prepare as much of the following information as possible:

• 15-minute or 30-minute load data
• Maximum grid demand
• Daily and monthly energy consumption
• Electricity tariff information
• Peak and off-peak price periods
• Demand charges, where applicable
• Existing PV capacity
• PV generation profile
• Backup-power requirements
• Desired peak-demand target
• Available charging periods
• Planned future loads
• Site electrical information

The more complete the data, the easier it becomes to determine whether the project requires 50kW, 100kW, 125kW or another power level, and whether the energy capacity should be 100kWh, 200kWh, 258kWh, 500kWh or larger.

Frequently Asked Questions

How do you calculate the required power of a C&I BESS?

A basic peak-shaving calculation is:

Required Battery Power = Current Peak Demand − Target Grid Demand

For example, if the site reaches 320kW and the target is 220kW, approximately 100kW of battery power may be required during the peak.

How do you calculate the required battery energy?

A simple theoretical estimate is:

Required Energy = Battery Power × Discharge Duration

If 100kW is required for two hours, the theoretical energy requirement is approximately 200kWh.

Is daily energy consumption enough to size a C&I battery?

No. Two sites with the same daily kWh consumption may have completely different peak-demand patterns. Load shape, peak duration and timing are critical for proper sizing.

Why is load-profile data important?

Load-profile data shows when peaks occur, how high they are and how long they last. It also reveals whether several peaks occur during the same day.

How does solar PV affect BESS sizing?

Solar reduces the site’s net grid demand and may also provide energy for battery charging. The battery should therefore be evaluated together with the PV generation profile.

Can one BESS provide both peak shaving and backup power?

Potentially, yes. However, sufficient battery energy may need to remain reserved for backup, which reduces the amount available for daily energy-management applications.

Is a larger battery always better?

No. Oversizing increases project cost and may reduce system utilization. The goal should be to match battery power and energy capacity to the site’s actual operating requirements.

Conclusion

Sizing a commercial and industrial battery energy storage system begins with understanding the site’s load profile.

The key questions are not simply how many kilowatt-hours the battery can store.

The more important questions are:

How high is the peak?

How much of that peak needs to be reduced?

How long does the reduction need to last?

How many peaks occur each day?

Can the battery recharge between peaks?

How much solar generation is available?

Does the system also need to preserve energy for backup?

From these questions, the basic sizing logic becomes much clearer.

The required kW is mainly determined by the amount of peak reduction needed.

The required kWh is mainly determined by how long that reduction must be maintained and how energy moves through the system during the day.

A well-sized C&I BESS therefore comes from analyzing power, energy, time and operating strategy together.

The goal is not to install the largest possible battery.

The goal is to select a system that is properly matched to the facility’s real load profile, electricity tariff, solar generation and long-term energy-management objectives.