Specifying a battery storage system

A Lastenheft (specification) is the point where an idea becomes a testable requirement. Stay too brief here and the resulting quotes differ formally but are barely comparable in substance. Go into technical detail without knowing your own constraints, and you lock in requirements no supplier can meet economically. A good specification for a battery storage system always covers the areas described below.

Performance data: usable capacity, not nameplate value

A storage system’s performance data covers usable capacity in kilowatt-hours, charge and discharge power in kilowatts, the resulting C-rate (charge or discharge power relative to capacity), round-trip efficiency, and permitted depth of discharge. It is essential that a specification asks for usable capacity, not the nameplate capacity of the installed cells, because the gap between the two can be substantial depending on the reserve and ageing strategy chosen. Expected degradation over the system’s life, in percent of usable capacity per year, belongs in the requirement too, not just a blanket guarantee figure at the end of the term.

Warranty by component

A storage system is made of several components with different service lives, typically battery cells, inverter, battery management system, and auxiliary equipment such as air conditioning or fire protection technology. A specification should ask for a separate warranty period for each component and, where relevant, a cycle life or calendar life guarantee, rather than a single blanket system warranty. Only this way can it later be established which component carries which period in the event of a defect.

Availability, grid code and fire protection

The required availability of the system over the year belongs in the specification as a measurable figure, as does compliance with the relevant grid connection rules, such as VDE-AR-N 4105 for a low-voltage or VDE-AR-N 4110 for a medium-voltage connection, depending on the connection point and power. Fire protection needs a section of its own. It covers requirements for compartmentation, extinguishing concept, gas detection, and coordination with the local fire brigade and the insurer, settled before the system is ordered, not at acceptance.

For the safety technology itself, a reference to the relevant standard series for stationary energy storage systems is advisable, such as the DIN EN IEC 62933 series, which sets terms, test methods and safety requirements for such systems. A specification that refers to this standard series, instead of drafting its own safety criteria from scratch, reduces the risk that a bidder simply overlooks a relevant requirement.

The grid connection itself belongs in the specification as its own sub-point: voltage level, permitted connection power, and who is responsible for registering with the grid operator should be recorded there, as should who coordinates the metering concept with the metering point operator. A specification that leaves this interface open pushes a coordination step that usually takes weeks into a phase of the project where delays are most expensive.

Interfaces, monitoring and operation

A specification sets out which interfaces the system must provide to the higher-level control, to the grid operator and to the operator’s own monitoring, in what format data is delivered and at what time resolution. Added to this are requirements for ongoing operation: who takes on maintenance and fault clearance, within what response times, and what spare-part availability is committed for over the contract term. If these points are not fixed beforehand, the gaps usually only show once the system is in operation.

Acceptance criteria and documentation

The specification should anticipate the later acceptance criteria: which capacity test is carried out, what tolerance between the committed and the measured power is accepted, and what documentation the installer must hand over, from wiring diagrams through the commissioning record to the electrical safety test records. Anyone who negotiates these criteria only after signing the contract gives up the strongest position they had in the process.

The duty to document during ongoing operation also belongs in the specification, not only the documentation for acceptance. This includes a maintenance schedule with fixed intervals, a procedure for reporting and resolving faults with clear deadlines, and how changes made to the system during operation are documented and added to the as-built records. A specification that fixes these requirements from the start prevents the documentation from drifting apart over the years until the system’s actual condition can no longer be reconstructed. Without this, the operator ultimately bears the risk of incomplete documentation, even though it could have been required contractually.

Common gaps

• A test phase too short to capture seasonal load patterns or rare operating states.
• A silent assumption of black-start capability or island-mode operation, without the hardware this requires being asked for in the specification.
• Missing or too generally worded warranty conditions for individual components, instead of a differentiated arrangement.
• No clear assignment of responsibility between installer, grid operator and operator for faults at the interface between them.

A bill of quantities can add the concrete breakdown by trade and item that bidders calculate their quotes from, but it does not replace the substantive depth of the technical requirements themselves. A solid specification comes from combining a company’s own load-profile data with these technical and contractual requirements, before a tender goes out. As of September 2026.

Sources

1. VDE-AR-N 4105, low-voltage technical connection rule, vde.com
2. VDE-AR-N 4110, medium-voltage technical connection rule, vde.com
3. DIN EN IEC 62933-1 (terminology for energy storage systems), DIN Media