How to Specify a Custom Battery for a Solar PV Energy Storage System

A Solar PV energy storage project does not become a suitable battery specification simply by converting PV capacity into battery capacity.

For OEMs, solar EPCs, system integrators, and energy storage solution providers, the battery needs to match the project's load profile, storage objective, inverter or PCS, installation environment, and target market. When standard products cannot meet those requirements, the next step is to define a custom battery specification that a manufacturer can evaluate, prototype, and produce.

This guide explains how to move from project requirements to a manufacturer-ready specification and RFQ.

Step 1: Define What the Battery Needs to Do

Start with the application, not the battery model.

Determine how the battery will be used and what the Solar PV system is expected to achieve. For example, a project may use the battery to increase solar self-consumption, reduce peak demand, provide backup power, or support an off-grid application. Each use case creates different requirements for battery capacity, power, operating profile, and controls.

The basic project information should include the PV capacity, typical and peak load, critical load if backup is required, desired storage duration, existing or planned PV configuration, and installation environment. For an existing system, also identify the current inverter or PCS and whether the battery will be connected through an AC- or DC-coupled architecture.

Consider a C&I project with 500 kWp of PV, a 350 kW peak load, and a two-hour backup requirement. Those figures provide a starting point for battery sizing, but they do not by themselves determine the final battery capacity.

The important output from this step is a clear description of the battery's job within the system. That becomes the basis for the electrical requirements in the next step.

Step 2: Define the Required Energy and Power

The battery specification needs two separate targets: usable energy in kWh and power in kW.

For backup applications, a useful starting calculation is:

Required usable energy ≈ critical load × required backup duration

If the critical load is 200 kW and the required backup duration is two hours, the system needs approximately 400 kWh of usable energy. The required nominal battery capacity will generally be higher because the design also needs to account for DoD, conversion losses, operating conditions, and the required performance over the battery's service life.

For solar self-consumption or peak shaving, the calculation needs to reflect the site's actual operating profile. The battery may charge when PV generation exceeds the load and discharge during selected periods. In these applications, hourly load and PV generation data can provide a much better basis for sizing than PV capacity alone.

Power must then be checked independently. If the system needs to discharge at 200 kW, a battery with 400 kWh of usable energy provides approximately two hours of theoretical discharge duration at that power:

Storage duration = usable energy ÷ discharge power

The actual design still needs to account for inverter/PCS limits, battery operating limits, and system losses.

At the end of this step, the project should have a defined usable energy target and continuous power target. These become the primary electrical requirements for the custom battery.

Step 3: Check the Inverter, PCS and BMS Requirements

The battery cannot be specified independently from the equipment it needs to work with.

For a new system, confirm the planned inverter or PCS requirements before finalizing the battery. For a retrofit project, these requirements are even more important because the existing equipment may impose fixed limits.

Check the inverter or PCS model, DC voltage operating range, maximum charge and discharge current, battery-side requirements, and communication interface. The BMS may need to communicate with the inverter, PCS, or EMS through CAN, RS485, or another specified protocol.

This is not simply a matter of confirming whether two products use the same connector. The battery's voltage range, current capability, BMS logic, protection limits, and communication behavior all need to be compatible with the system.

For example, if the PCS requires the battery to operate within a specific DC voltage range, the battery configuration must be designed around that range. If the PCS depends on BMS communication to control charging and discharging, the required communication protocol and data exchange need to be defined before the battery design is finalized.

For detailed inverter-battery matching considerations, a project can also refer to ACE Battery's existing guide on How to Match a Solar Inverter with a LiFePO4 Battery.

The result of this step is a defined set of electrical and communication constraints that the custom battery must satisfy.

Step 4: Decide What Actually Needs to Be Customized

Once the system requirements are clear, determine whether a standard battery can meet them.

A standard product may be suitable when its voltage, capacity, power, dimensions, communication interface, and inverter compatibility already match the project. Customization becomes relevant when one or more critical requirements fall outside the standard product configuration.

For example, customization may be considered when the project requires a specific voltage or capacity, has strict space limitations, needs particular BMS functions, requires integration with a specific inverter or PCS, or needs a different mechanical configuration.

The same principle applies to OEM and private-label projects. A battery may meet the electrical requirements but still require changes to its product configuration or manufacturing process.

The objective is therefore not to customize as many parameters as possible. It is to identify which requirements cannot be met by a suitable standard product and customize only where necessary.

This distinction can reduce unnecessary engineering work while keeping the battery aligned with the actual application.

Step 5: Build the Custom Battery Specification

After identifying the required customization, turn the project requirements into a single technical specification.

The specification should define the parameters that the manufacturer needs to design against, rather than simply describing the project in general terms.

At the electrical level, specify the battery chemistry, nominal voltage, usable capacity, rated charge/discharge power, DoD, and required cycle-life target.

At the integration level, define the inverter or PCS, operating voltage range, communication interface, BMS requirements, and EMS requirements.

Mechanical and environmental requirements should cover the available installation space, maximum dimensions, installation configuration, operating temperature, and enclosure requirements where applicable.

Finally, define the target market and the compliance requirements that apply to the intended battery and system configuration.

A useful specification might therefore read more like:

C&I Solar PV application / LFP / defined nominal voltage / defined usable capacity / defined continuous power / specified DoD / CAN communication with target PCS / outdoor installation / defined dimensional limits / target market requirements

than simply:

500 kWp solar system + 1 MWh battery

The first gives a manufacturer an engineering starting point. The second provides only a project-level description.

The specification should also distinguish between mandatory requirements and preferred requirements. This gives the manufacturer room to propose alternative configurations where they can achieve the same project objective more effectively.

Step 6: Turn the Specification Into a Custom Battery RFQ

Once the technical specification is ready, it can become the basis of the custom battery RFQ.

The RFQ should not repeat the entire project discussion. Instead, use the finalized specification as the technical baseline and add the information a manufacturer needs to assess development and production.

This normally includes the required prototype quantity, expected production volume, target development schedule, desired mass-production date, and any requirements for validation or certification support.

For example, a buyer might request:

Develop and quote a custom LFP battery for a C&I Solar PV application based on the attached electrical and mechanical specification. The battery must communicate with the specified PCS, meet the defined operating range, and be validated before pilot production.

This gives the manufacturer a much clearer basis for evaluating feasibility and cost than a general request for a “custom solar battery.”

When comparing proposals, focus on whether the manufacturer can meet the defined technical requirements, support the required integration and validation, and scale the design from prototype to production.

For OEMs and system integrators looking for a long-term solar battery OEM/ODM partner, this is also the point where development capability becomes as important as the initial product quotation.

Have a project specification ready? ACE Battery can work with OEMs, system integrators, and energy storage solution providers to evaluate custom battery requirements and development needs.

Step 7: Validate the Battery Before Mass Production

A manufacturer quotation and prototype do not confirm that the battery is ready for production. The prototype needs to be tested against the requirements established during the specification stage.

Start with electrical validation. Confirm the battery's actual capacity, voltage range, charge/discharge performance, power capability, and protection functions.

Then test system integration with the intended inverter or PCS. BMS communication, alarms, operating limits, and charge/discharge controls should be verified under representative conditions.

Additional environmental, enclosure, safety, and compliance testing may be required depending on the application, target market, and final system configuration.

The development process should therefore follow a controlled sequence:

Specification → Prototype → Validation → Pilot Production → Mass Production

The purpose of validation is not only to confirm that the battery works. It is to confirm that it works within the intended Solar PV energy storage system and according to the agreed specification.

For OEM and ODM projects, identifying design or integration issues before pilot or mass production can help avoid later changes to the battery design, tooling, certification work, and production process.

From Solar PV Project Requirements to a Custom Battery

A custom battery project is easier to manage when each stage produces the information required by the next one:

Project application → Energy and power requirements → Inverter/PCS constraints → Customization decision → Battery specification → RFQ → Prototype validation → Production

This approach avoids treating battery capacity as the only design parameter. It also gives the manufacturer enough technical context to evaluate the battery as part of the complete energy storage system.

For OEMs, solar EPCs, system integrators, and energy storage brands, ACE Battery provides custom battery and energy storage development for application-specific requirements.

If you are developing a Solar PV + ESS project and need a battery designed around your system, contact ACE Battery with your project requirements to discuss specification, prototype development, and production.