Accepting a battery storage system before final payment

Abnahme (acceptance) is the moment a completed system becomes, legally and technically, the system the contract promised. From here the warranty starts running, and the final payment falls due. Anyone who treats this step lightly takes on risks that should really sit with the installer.

Documents before the walkthrough

Before the actual walkthrough come the documents. These include the as-built documentation of components actually installed, wiring diagrams, the fire protection and ventilation concept, the electrical safety test records under DIN VDE 0100-600, and proof of grid-connection compliance, for example under VDE-AR-N 4105 or VDE-AR-N 4110. If any of these documents is missing, the later walkthrough cannot be completed, because the basis for comparison is missing. These documents should be available at least a week before the planned acceptance date, so they can be checked in advance rather than reviewed for the first time during the walkthrough itself.

Visual and safety inspection

The walkthrough itself starts with a visual inspection of the system: labelling, access, clearances, cable routing, switchgear cabinet labelling and the condition of enclosures. The safety inspection runs in parallel and covers earthing, insulation resistance and the function of protective devices, among other things. These checks decide whether the system may even go into trial operation, before any function test begins.

Whoever carries out the visual and safety inspection should be independent of the installation itself, so the result is not shaped by the installer’s own preparatory work. In practice this role is taken by a qualified electrician acting either for the operator or on behalf of an independent client representative, not the installer that built the system.

Function tests and the commissioning record

The function tests check how the battery management system, inverter and higher-level control work together under defined load cases, including charge and discharge cycles and behaviour under simulated fault conditions such as a grid outage, where island mode or emergency-power function was contractually agreed. The result is recorded in the commissioning record, together with the log files of the battery management system and the controller for the entire test period. These records are the reference later warranty claims rest on.

The capacity test

The capacity test measures the system’s actually usable capacity under controlled conditions and compares it with the contractually committed value. It is the central quantitative evidence of the acceptance, because it draws directly on the performance commitment in the specification. A deviation within the contractually agreed tolerance counts as met; a deviation beyond it counts as a defect, to be corrected or contractually compensated before full acceptance.

Defect classes and acceptance subject to reservation

Not every finding blocks acceptance. Common practice is to sort findings into defect classes: safety-relevant defects that must be fixed before commissioning, functional defects that limit operation without posing a danger, and minor defects that do not affect use. Acceptance subject to reservation lets the operator take over the system and start the warranty while recorded defects are fixed within an agreed period. This distinction protects the operator without placing an unreasonable burden on the installer.

For each defect class the acceptance record should set its own deadline for correction, plus the person or body who then checks and confirms that correction. Without this, it stays unclear who checks when whether a recorded defect was actually fixed, and an open item can drag on for months without either side formally responsible for it. A second, shorter follow-up walkthrough after the agreed deadline closes this loop and records in the protocol which defects genuinely count as resolved.

Handover and the start of the warranty

Acceptance also transfers responsibility for ongoing operation. This includes a complete operating manual describing operation, maintenance intervals and fault behaviour, and a list of spare parts to be held for the contractually agreed response time. If this handover is missing or incomplete, the operator is left without information it could have required contractually the first time a fault occurs. Access credentials for monitoring and control, and a list of all maintenance and fault-reporting contacts, belong in this handover package too.

The start of the warranty period is tied to the date of formal acceptance, not to actual commissioning or the first day of operation. This distinction should be fixed in the contract, because weeks often pass between first commissioning and formal acceptance while open items are resolved. Only once the acceptance record is signed does the period on which later claims rest start running.

Because the installer has its own interest in a quick acceptance while the operator has an interest in a complete and reliable check, this step benefits from a party that is bound to neither interest. Independent owner’s engineering, meaning technical representation of the client, accompanies exactly these steps, so acceptance happens in the operator’s interest. As of September 2026.

Sources

1. DIN VDE 0100-600 (verification of electrical installations), DIN Media
2. VDE-AR-N 4105, low-voltage technical connection rule, vde.com
3. VDE-AR-N 4110, medium-voltage technical connection rule, vde.com