Home Battery Storage: What a Proper Install Looks Like

Home battery storage is the install type I get called in on more than any other at the moment. In a typical week I have two or three conversations with homeowners who have had an estimate and want a second pair of eyes before they commit, usually a retrofit onto existing solar, increasingly part of a package that also takes in a heat pump or EV charger. The retrofit market has grown faster than its install standards, and the gap shows up in specific places against BS 7671:2018+A4:2026 and the IET Code of Practice for Electrical Energy Storage Systems.

What a home battery actually is

A domestic battery energy storage system (BESS) consists of three elements: the battery cells themselves, a battery management system (BMS) that monitors each cell's voltage, temperature and state of charge, and an inverter that converts between the battery's DC side and the home's 230 V AC side. The battery and BMS are usually packaged as a single unit in a wall-mounted or floor-standing enclosure. The inverter is either integrated with the battery or a separate wall-mounted box.

The install splits into two broad architectures:

• AC-coupled. The battery has its own dedicated inverter that connects to the consumer unit on a new circuit. An existing solar PV array keeps its existing solar inverter and feeds the consumer unit as before. The battery inverter is told by a smart meter or CT clamp when the house is exporting power and diverts it into the battery. Simpler retrofit, slightly lower round-trip efficiency.
• DC-coupled (hybrid inverter). A single inverter handles both the solar PV DC input and the battery DC input, with a single AC output into the consumer unit. Tidier, usually more efficient, but requires replacing the existing solar inverter on a retrofit, which disturbs the original solar MCS certification and may affect the Smart Export Guarantee tariff.

What A4:2026 brought to this

Amendment 4 did not rewrite the rules for BESS installations. It pulled the main BS 7671 text closer to the IET Code of Practice, which has been the authoritative document since 2017 but was not always referenced directly in domestic installer training. Tighter now:

• Residual current protection selection. The battery circuit needs correct-type RCD protection. Type A is the minimum default for most BESS inverters; Type B may be required depending on the inverter topology. The installer has to read the inverter datasheet and specify against it, not default to Type AC.
• Surge protection at the origin. A4:2026 makes Type 2 SPD the default at the incoming position of most new domestic installations. For a property with a battery, it is effectively mandatory.
• Isolation and labelling. The battery needs a clearly-labelled, easily-accessible isolation means. The consumer unit must carry a warning label indicating on-site generation and storage is present. Every notifiable final circuit is to be labelled.
• Fire-safe separation. Batteries should be fitted where an internal fault cannot block an escape route or pass heat into a habitable room, with a fire-rated barrier where the location does not provide one by default.

What I see go wrong on site

In rough order of how often I find each one.

Battery fitted in the wrong room

The cheapest install puts the battery on whichever wall is nearest the existing consumer unit, regardless of what room that wall is in. I have found them in bedrooms, in cupboards under stairs that form part of a protected escape route, and against unprotected timber-frame walls. The IET Code of Practice is specific on siting. The preferred order is an external non-combustible enclosure, an integrated garage or outbuilding, then a utility or plant room. Anywhere else needs a fire-safe assessment and usually a fire-rated barrier.

No Northern Powergrid notification

The DNO needs to know what generation and storage is sitting on their network. G98 covers single-phase devices up to 16 A per phase, connect-and-notify, so the installer can energise and submit the notification after the fact. G99 covers larger devices or combinations, prior approval required. I have been called to properties where the battery has been running for months with nothing on file at Northern Powergrid. It is non-compliant, it breaks SEG eligibility, and a buyer's solicitor will pick it up at conveyancing.

Wrong-type RCD protection

Battery inverters have specific RCD requirements that installers do not always read. A Type AC RCD on an inverter circuit specified for Type A or Type B is a non-compliance. The inverter will run. In a fault it may not disconnect in the way it was designed to.

No isolation means at the battery itself

The fire service needs to be able to isolate the battery at the battery, not only at the consumer unit. A clearly-labelled, easily-accessible isolator at the unit is required under both BS 7671:2018+A4:2026 and the IET Code of Practice. Missing it is a straightforward fail on an EICR.

No surge protection at the origin

A battery inverter is £1,500 to £3,500 of sensitive power electronics. A transient from a nearby lightning strike or a switching event on the grid can take it out in microseconds. A Type 2 SPD at the origin costs around £100 fitted and is now the default expectation under A4:2026 for any domestic installation carrying generation or storage.

Existing board not checked

A battery changes the load picture in both directions. Tails, main earthing conductor, bonding to gas and water, RCD or RCBO selectivity, all of it has to be confirmed against the new installation. Bolting a new battery circuit onto a tired 2005 plastic consumer unit without opening the front and reading the test results is where nuisance tripping and EICR C2 findings start.

No handover documentation

A compliant install leaves the homeowner with an Electrical Installation Certificate, an MCS certificate where applicable, the inverter commissioning record, proof of DNO notification, a labelled wiring diagram and a written emergency procedure. A photo on WhatsApp is not a handover.

The electrician's scope versus the installer's scope

On an MCS-certified install, the solar or heat-pump firm holds the scheme membership and issues the MCS certificate. The electrical work around the battery, the final circuits, any consumer unit modifications, the DNO notification and the EIC, is typically done either in-house by the firm's own electrician or by an NICEIC-registered electrician working alongside them.

The retrofit market is a different picture. Plenty of firms selling retrofit batteries are not MCS-registered. Home insurers have started asking for the paperwork before they pay out on any fire claim: an MCS certificate where applicable, or an EIC and DNO notification from a registered contractor. A battery fitted by a general-trade firm with no registration trail is hard to underwrite after the event.

On a lot of these jobs the lead firm handles the MCS side and I handle the electrical side: consumer unit, final circuit, RCD selection, SPD, labelling, Northern Powergrid notification, and the EIC. Both documents sit with the homeowner. Both can be produced at sale or at a claim.

What to ask before signing

1. Is the install MCS certified, and can I see the certificate number?Check it on mcscertified.com before signing.
2. Which registered scheme is the electrical work under?NICEIC, NAPIT, ECA or Stroma. If the answer is none, walk away.
3. Where exactly will the battery go, and why is that location suitable?Get a specific answer against the IET Code of Practice. Not a vague "the installer will decide on the day".
4. Is a consumer unit upgrade included or extra?Get this confirmed before signing, not invoiced afterwards.
5. What is the DNO notification process, G98 or G99?And who submits it, the installer or the homeowner?
6. What documents will I receive at handover?EIC, MCS certificate, commissioning record, DNO notification, emergency procedure. All of them, on paper or PDF.
7. Who do I ring in 2 years when the inverter needs a firmware update or a fault?The installer or the manufacturer? Get the answer before the warranty becomes theoretical.

What I get called in to fix

Roughly once a month I am asked to sort out a bad battery install. Common findings:

• Nuisance tripping on the inverter circuit, traced to wrong-type RCD selection.
• A consumer unit running hot because the tails and earthing were never upgraded for the combined load.
• A property on the market where the buyer's solicitor has asked for the battery paperwork and the seller has a brochure and an invoice, no EIC, no DNO notification.
• An insurance claim refused after a fire because the install was not MCS and the homeowner cannot evidence that the electrical work met BS 7671.

All of it is avoidable at the specification stage. The gap between a compliant install and a cut-price one is usually £300 to £800, and that covers the paperwork, the correct RCD, the SPD, the isolator, the consumer unit check and the DNO notification. Skip any of it and when something does go wrong, it is the homeowner holding the bill, not the installer.