How to Size a Battery Energy Storage System (BESS) for Your Project
Understanding the Fundamentals of BESS Sizing
Battery Energy Storage Systems (BESS) are becoming an increasingly important part of commercial, industrial, and utility-scale energy projects. Whether the goal is reducing demand charges, increasing energy resilience, supporting solar generation, or creating a microgrid, proper battery sizing is critical to achieving the desired financial and operational outcomes.
One of the most common misconceptions is that battery sizing is simply a matter of choosing the largest battery the budget allows. In reality, a properly designed BESS should be tailored to the facility’s energy profile, project objectives, utility rate structure, and future growth plans.
This guide explains the key concepts involved in sizing a Battery Energy Storage System and highlights the factors developers, EPC contractors, and facility owners should consider before selecting a solution.
Understanding Power vs. Energy
Before sizing a battery system, it’s important to understand the difference between power and energy.
Power (kW or MW)
Power refers to the rate at which electricity can be delivered.
For example:
- 500 kW battery
- 1 MW battery
- 5 MW battery
Power determines how much load the battery can support at a given moment.
Energy (kWh or MWh)
Energy refers to the total amount of electricity stored in the battery.
Examples include:
- 1 MWh battery
- 2 MWh battery
- 10 MWh battery
Energy determines how long the battery can provide power.
Example
A battery rated at:
500 kW / 2 MWh
can theoretically provide:
- 500 kW for 4 hours
- 1 MW for 2 hours (if inverter capacity allows)
- 250 kW for 8 hours
Both power and energy ratings must be considered during the design process.
Step 1: Define the Project Objective
The first step in BESS sizing is identifying the primary goal of the system.
Different applications require different sizing approaches.
Peak Shaving
Peak shaving systems reduce utility demand charges by discharging during periods of highest facility demand.
The battery is typically sized based on:
- Peak demand levels
- Duration of demand spikes
- Utility tariff structure
Backup Power
Backup power applications focus on maintaining critical loads during utility outages.
Sizing is based on:
- Critical load requirements
- Desired backup duration
- Reliability objectives
Solar Energy Shifting
Solar-plus-storage projects use batteries to store excess solar production and discharge when solar generation decreases.
Sizing depends on:
- Solar production profile
- Load profile
- Utility rate structure
Microgrid Applications
Microgrids require batteries that support both reliability and economic optimization.
These projects often involve more detailed modeling and multiple operating scenarios.
Step 2: Analyze the Facility Load Profile
A battery should never be sized using monthly utility bills alone.
Instead, designers should review interval data whenever possible.
Useful data includes:
- 15-minute interval data
- Hourly consumption data
- Demand peaks
- Seasonal variations
- Critical load requirements
Understanding how electricity is consumed throughout the day is essential for identifying battery opportunities.
Example Load Profile
A manufacturing facility may have:
- Average demand: 900 kW
- Peak demand: 1.8 MW
- Peak duration: 3–4 hours
This information helps determine the required battery power and energy capacity.
Step 3: Determine the Required Power Rating
The power rating determines how much demand reduction the battery can provide.
Example
Facility peak demand:
1.8 MW
Target demand reduction:
500 kW
Recommended battery power rating:
500 kW
In this scenario, the battery would discharge during peak demand periods to reduce the facility’s utility demand charge exposure.
Step 4: Determine the Required Energy Capacity
After determining the power requirement, the next step is calculating the energy capacity needed.
Example
Battery discharge requirement:
500 kW
Peak duration:
4 hours
Required energy capacity:
500 kW × 4 hours = 2,000 kWh
or
2 MWh
Recommended system:
500 kW / 2 MWh
This configuration allows the battery to support the targeted demand reduction for the full peak period.
Step 5: Account for Battery Efficiency and Degradation
Real-world battery systems do not operate at 100% efficiency.
Factors that impact performance include:
- Round-trip efficiency losses
- Temperature effects
- Battery aging
- Depth of discharge limitations
Many lithium-ion systems achieve round-trip efficiencies between 85% and 95%.
Designers often include additional capacity to account for long-term degradation and maintain performance throughout the project’s life.
Step 6: Consider Future Facility Growth
One of the most common design mistakes is sizing a battery only for current operating conditions.
Future considerations may include:
- Facility expansion
- Additional production equipment
- EV charging infrastructure
- Electrification initiatives
- Increased HVAC loads
Planning for future growth can help avoid costly system upgrades later.
Common BESS Sizing Mistakes
Sizing Based Only on Monthly Utility Bills
Monthly bills rarely provide enough detail to accurately size a battery system.
Interval data is significantly more valuable.
Ignoring Peak Duration
Reducing a peak for 15 minutes requires a different battery than reducing a peak for four hours.
Oversizing the System
Larger batteries are not always better.
Oversizing can increase project costs and reduce return on investment.
Undersizing the Inverter
A battery may have sufficient energy capacity but still lack adequate power output if the inverter is undersized.
Ignoring Future Loads
Many facilities underestimate future energy requirements, leading to premature system constraints.
Example: Cold Storage Facility
Consider a 150,000-square-foot refrigerated warehouse.
Facility characteristics:
- Peak demand: 2 MW
- Demand charge: $25/kW
- Target reduction: 500 kW
- Peak duration: 4 hours
Potential battery recommendation:
500 kW / 2 MWh BESS
Benefits may include:
- Lower monthly demand charges
- Improved energy resilience
- Better utilization of on-site solar generation
- Reduced operating costs
A detailed engineering analysis would be required to optimize final system sizing.
How Solar and Battery Storage Work Together
Battery storage becomes even more valuable when paired with solar generation.
Benefits include:
- Storing excess solar production
- Increasing self-consumption
- Time-of-use optimization
- Improved resilience
- Reduced grid dependence
Many commercial facilities are now evaluating solar and storage together rather than as separate investments.
The Bottom Line
Proper BESS sizing requires more than selecting a battery based on available budget or nameplate capacity.
Successful projects begin with a thorough understanding of:
- Facility load profiles
- Project objectives
- Utility tariffs
- Future growth plans
- Solar generation opportunities
A properly sized Battery Energy Storage System can deliver significant savings, improved resilience, and stronger project economics over the life of the system.
How AmeriSol Energy Solutions Can Help
AmeriSol Energy Solutions supports developers, EPC contractors, and facility owners by helping source battery storage equipment for commercial, industrial, and utility-scale applications.
We can assist with:
- Solar modules
- Commercial inverters
- Battery Energy Storage Systems (BESS)
- EV charging infrastructure
- Balance-of-system equipment
- Procurement support for large commercial projects
Whether you’re evaluating peak shaving, backup power, solar integration, or microgrid applications, our team can help identify the right equipment solutions for your project. Our team can also assist customers evaluating:
- Domestic content options
- FEOC-compliant equipment
- Product availability
- Procurement strategies
- Alternative equipment solutions
For equipment pricing and availability, please complete our Request Pricing & Availability Form.
For project consultation and equipment sourcing assistance, please complete our Free Consultation Form.
Contact AmeriSol Energy Solutions
Website: https://american-solar.com
Email: contact@american-solar.com
Phone: (929) 376-0807
References
National Renewable Energy Laboratory (NREL):
https://www.nrel.gov
U.S. Department of Energy:
https://www.energy.gov
Sandia National Laboratories:
https://www.sandia.gov
Battery Energy Storage System Guidebook:
https://www.energy.gov/eere/solar