MIS 3012 battery storage standard

MIS 3012:2025 is the MCS Battery Installation Standard. It specifies what an MCS contractor must do when supplying, designing, installing, setting to work and commissioning a qualifying electrical energy storage system in a permanent building.

As at 22 July 2026, the current published issue was MIS 3012:2025 Issue 1.0, dated 1 January 2025. Its scope covers the listed behind-the-meter use cases up to a maximum system power output of 50kW. The version and scope should be checked again whenever an installation is being contracted.

What sits inside and outside the standard

The in-scope arrangements include battery systems connected behind the consumer’s meter through a dedicated inverter, through a hybrid inverter shared with generation, or without another generator on site.

The exclusions include several materially different systems, such as primary batteries, vehicle traction and starter batteries, mobile units, installations connected in front of the meter, systems without a permanent and normally live distribution-network connection, and certain UPS, emergency and product-integrated batteries.

An exclusion does not mean that no standards apply. It means MIS 3012 is not the document that certifies that use case.

MCS certification is a defined scope

MIS 3012 says compliance is mandatory for contractors certified to MCS:2025 for battery work. Battery and solar PV are separate certification scopes. An MCS solar listing does not confer battery scope.

The MCS register identifies the certified legal entity and technology scope. The installation certificate identifies the installed system and contractor, which may differ from the company that sold the package.

MCS is a certification scheme, not a substitute for statutory electrical and building duties. Conversely, an installation being electrically safe does not mean it can be represented as MCS certified when the contractor and work do not meet the scheme requirements.

The standards hierarchy is explicit

MIS 3012 requires the battery system to be designed and installed to the latest IET Code of Practice for Electrical Energy Storage Systems. If requirements conflict, it gives this order of precedence:

1. the latest BS 7671;
2. the applicable MIS 3012 requirements; then
3. the IET EESS Code of Practice.

The MIS also tells users to apply the latest edition of referenced standards unless it specifies a dated version. This prevents an old reference printed in the document from freezing the installation at that edition.

Manufacturer instructions remain part of the design. Where they conflict with the MIS, the contractor must resolve and document the issue rather than choosing whichever instruction is easiest to follow.

Packaged and composite systems allocate responsibility differently

MIS 3012 classifies systems according to how the battery, power-conversion equipment, controls and any islanding arrangement are packaged. A system supplied as a matched package leaves more product-level integration responsibility with its manufacturer. A composite system assembled from separate manufacturers’ components places more system-design responsibility on the installer.

This distinction matters for compatibility, protection, control, warranty and fault behaviour. “Each component is certified” is not evidence that a mixed system works safely as a complete installation. The design record should identify the class and the party responsible for confirming the combination.

Grid connection uses the combined installation

The contractor must follow the appropriate G98 or G99 process and G100 where export limitation is used. MIS 3012 makes clear that the 16A-per-phase threshold is based on the aggregated AC output of generation connected in parallel, not on each inverter viewed in isolation.

An existing PV inverter and a new AC-coupled battery can therefore change the connection route. The contractor should record all existing generation, the proposed equipment, its Registered Capacity and any export-limitation arrangement before selecting the application process.

Safety has to be verified in every operating mode

The standard requires protection against electric shock, overcurrent and overload in every intended operating state. For a backup system, that includes island mode, when the public supply is disconnected and the inverter becomes the source for maintained circuits.

The design must coordinate isolation, neutral-earth arrangements, the consumer earth electrode and protective devices. It must also address battery enclosure protection, access, DC-rated equipment, controls, maximum depth of discharge and the extra responsibilities that apply to composite systems.

MIS 3012 should not be used as a do-it-yourself wiring guide. Its requirements depend on calculations, equipment data, the existing installation and tests performed by competent people.

Location, fire and structural checks

MIS 3012 requires safe access, suitable ventilation, structural support, protection from foreseeable damage and flooding, and fire detection where appropriate. It also requires the latest BS 7671 to take precedence.

For a battery in a dwelling, PAS 63100 is now part of the current standards chain. During the 2026 BS 7671 transition, the contractor should identify which BS 7671 edition is being used and document the PAS assessment rather than relying on an old location rule.

Commissioning produces more than one certificate

The system must be commissioned against a documented procedure and shown to work as designed. MIS 3012’s example checklist covers system inspection, BS 7671 tests, equipment and controls, grid connection, export limitation, metering and any backup function.

The handover includes, as applicable:

• the MCS certificate;
• the BS 7671 electrical certificate and schedules;
• G98 acknowledgement or G99/G100 approval records;
• a system schematic for every operating mode;
• commissioning results, including backup changeover where provided;
• product declarations, serial numbers and warranties;
• fire, shutdown and isolation information; and
• operating, maintenance and data-access instructions.

The MCS certificate does not replace those technical records. It sits alongside them.

Related entries

• MCS
• MCS installation standards
• MIS 3002 solar PV standard
• IET EESS code of practice
• BS 7671 IET Wiring Regulations
• G99 application pathway
• Island mode and anti-islanding
• Whole-home backup changeover
• PAS 63100 domestic battery fire specification