How to Design a C&I Rooftop Solar + Battery Storage System

Designing a C&I rooftop solar + battery storage system starts with the project, not the battery.

A commercial or industrial site may have variable operating hours, production schedules, demand peaks, critical loads, and limits on grid import or export. These conditions determine how the PV system, battery storage, PCS, inverter, EMS, and electrical infrastructure should work together.

For EPCs, system integrators, and project developers, the design process should therefore move from the site requirements to the complete system architecture:

Site data → System objectives → PV design → Battery role → System architecture → Electrical integration → EMS → Site feasibility → System validation

This approach helps avoid selecting individual components before the requirements of the complete C&I system are clear.

1. Start With the C&I Site Load and Solar Profile

The first step is to establish how the site consumes electricity and when solar generation will be available.

Monthly electricity consumption provides a useful starting point, but it does not show when demand occurs. For C&I projects, the design team should examine the site's peak load, daily load profile, operating hours, weekday and weekend patterns, production schedules, and critical loads. For existing PV systems, the actual solar generation profile should also be included.

Where available, interval load data can reveal recurring demand patterns that are not visible in monthly consumption figures.

The purpose of this step is to establish the load and generation profile that will guide the rest of the system design.

2. Define What the Solar + Battery System Needs to Achieve

The next step is to define the project's operating objectives.

A C&I solar + battery system may be designed primarily for solar self-consumption, peak-demand reduction, time-of-use optimization, backup power, or a combination of these applications.

The priority matters because different objectives can lead to different system requirements. A site focused on solar self-consumption may need to shift excess daytime PV generation to later periods. A site focused on peak shaving may place greater importance on the battery's ability to respond to specific periods of high demand. If backup is required, the system also needs to preserve energy for critical loads.

The project team should therefore agree on the main operating objectives before selecting the system architecture.

The key question is: What should the battery help the site achieve?

This decision provides the basis for the PV and battery design that follows.

3. Match Rooftop PV Capacity With the Site Load

Once the project objectives are clear, determine how the rooftop PV system should contribute to them.

PV capacity should be assessed against the site's daytime electricity demand, available roof area, expected solar generation, seasonal load changes, grid export limits, and potential future demand.

The relationship between PV generation and site demand is particularly important. If PV production regularly exceeds on-site consumption, the project needs to determine whether the surplus can be stored, exported, or needs to be curtailed. If site demand remains high during most daylight hours, a larger share of PV generation may be consumed directly.

This is why a fixed PV-to-battery ratio is not appropriate for every C&I project. The PV system should first be evaluated in the context of the site's load and grid conditions. Battery storage can then be designed around the resulting energy flows.

The objective is to determine the appropriate role and scale of PV within the overall site energy system, not to size the battery at this stage.

4. Define the Battery's Role in the C&I System

After the PV role has been established, determine what the battery needs to do within the system.

The key requirements are power and usable energy. Power relates to how much electricity the battery needs to deliver or absorb at a given time, while usable energy relates to how much energy is available for the intended application.

For example, a battery supporting peak shaving needs sufficient discharge power to respond to the site's demand peaks. A battery used to shift excess solar generation may place greater emphasis on the amount of usable energy available over the required operating period.

Backup applications introduce another consideration: some energy may need to remain available for critical loads instead of being used for normal daily operation.

The required battery power and usable energy should therefore be determined by the site's operating objectives and load profile rather than by PV capacity alone.

For projects requiring a battery solution developed around specific application and integration requirements, see How to Specify a Custom Battery for a Solar PV Energy Storage System.

5. Choose the PV + Battery System Architecture

With the PV and battery roles defined, the next decision is how they should be connected within the electrical system.

AC-Coupled Architecture

In an AC-coupled configuration, the PV system and battery use separate power conversion equipment and connect on the AC side.

This can be useful for many retrofit projects because an existing PV array and inverter may be retained while a battery PCS is added to the site's AC system.

DC-Coupled Architecture

In a DC-coupled configuration, PV and battery storage are integrated on the DC side before power is converted to AC.

This approach can be considered when PV and storage are designed together and the architecture provides a suitable technical or economic advantage.

The choice between AC and DC coupling should consider the project's new-build or retrofit status, existing inverter configuration, energy flow requirements, control strategy, electrical infrastructure, and project economics.

For an existing rooftop PV system, the design team should first determine whether the existing equipment can accommodate the proposed storage architecture. For a new project, AC and DC configurations can be compared as part of the overall system design.

The architecture should follow the project requirements rather than being selected independently of the PV and battery design.

6. Check PCS, Inverter and Grid Integration

The selected architecture must then be checked against the site's electrical infrastructure.

The PCS, PV inverter, battery, transformer, AC system, and grid connection need to support the intended power flows under the project's operating scenarios.

For example, the design team should verify that the available inverter and PCS capacity is consistent with the required charging and discharging operation. The battery's operating voltage and power limits also need to be compatible with the selected power conversion equipment.

For retrofit projects, existing infrastructure can become an important design constraint. Transformer capacity, grid connection limits, existing PV inverter capacity, and permitted import or export levels may all affect how additional storage can be integrated.

Protection and communication requirements should also be considered as part of the system-level electrical design.

The important distinction is that battery energy capacity does not by itself determine the power capability of the complete system.

For detailed battery-to-inverter compatibility considerations, see How to Match a Solar Inverter with a LiFePO4 Battery.

7. Design the EMS Operating Strategy

After the electrical architecture is established, define how the system will operate under different conditions.

The EMS coordinates energy flows between PV generation, site loads, battery storage, and the grid.

For example, when PV generation exceeds site demand, the EMS may direct the available energy toward battery charging. During periods of high site demand, it can control battery discharge according to the project's operating objectives. If the project uses time-of-use optimization, the EMS can also incorporate defined operating periods and grid conditions.

Multiple objectives require particular attention. A strategy that uses the battery aggressively for peak shaving could reduce the energy available for backup. Similarly, maximizing solar self-consumption may not always align with a required backup reserve.

The EMS strategy should therefore define the operating priority and the conditions under which the battery charges, discharges, or maintains its SOC reserve.

The EMS translates the project's objectives into actual system behavior.

8. Check Site and Installation Constraints

The proposed system must also fit the physical and environmental conditions of the site.

The design should consider the available battery installation area, indoor or outdoor conditions, ambient temperature, thermal environment, cable routing, electrical room space, maintenance access, and applicable fire and electrical requirements.

Battery storage does not necessarily need to be installed on the rooftop. Depending on the project layout and local requirements, a dedicated ground-level area or suitable electrical space may provide a more practical installation location.

These constraints can affect equipment placement, cable routing, thermal management, and sometimes the overall system architecture.

The purpose of this step is to confirm that the proposed system can be installed, operated, and maintained under the actual site conditions.

9. Validate the Complete C&I Solar + Battery System

Before deployment, the entire system should be validated as one integrated solution.

The final design review should confirm that the PV system, battery, PCS, inverter, EMS, grid connection, and site infrastructure can operate together under the intended project scenarios.

Key validation points include:

• PV generation and site load assumptions

• Battery power and usable energy

• PCS and inverter compatibility

• EMS operating logic

• SOC reserve requirements

• Grid import and export limits

• Electrical protection

• Site and environmental conditions

A practical project sequence is:

System design → Equipment selection → Integration testing → Site commissioning → Performance validation

The goal is not simply to verify each component individually. It is to confirm that the complete Solar + Battery Storage system performs as intended under actual C&I operating conditions.

Conclusion

A C&I rooftop solar + battery storage system should be designed from the site and project objectives backward, rather than starting with a battery model or a fixed PV-to-storage ratio.

The overall design process is:

Site data → System objectives → PV design → Battery role → System architecture → Electrical integration → EMS → Site feasibility → System validation

This approach allows EPCs, system integrators, and project developers to design around the actual relationship between PV generation, site demand, battery operation, and grid requirements.

ACE Battery supports application-specific energy storage solutions for C&I projects, including customized battery development and system integration.

Planning a C&I rooftop PV + storage project? Contact ACE Battery to discuss your project requirements and energy storage solution.