Introduction
UK installers fitted over 40,000 certified battery storage systems in 2025, nearly double the previous record, and the document setting the technical bar for that work is the IET Code of Practice for Electrical Energy Storage Systems. It is not a law and not an MCS standard, yet it is the reference an assessor, an insurer or an expert witness will reach for when a battery installation is questioned. Understanding what it requires, and how it interlocks with MIS 3012, PAS 63100 and BS 7671, has become part of the job.
What the IET Code of Practice Covers
The Code of Practice, now in its 3rd Edition published in April 2024, covers the specification, design, installation, commissioning, operation and maintenance of electrical energy storage systems in fixed applications: individual dwellings, commercial and multi-occupancy buildings, and industrial sites, for both AC- and DC-coupled integration into low-voltage systems. In practice it is the bridge between BS 7671, which governs the electrical installation generally, and the specific behaviours of battery systems: charge and discharge regimes, island mode operation, protection coordination and battery management.
What Changed in the 3rd Edition
Two changes matter most on site. First, the requirements for island mode isolators were updated, and the guidance on fire safety and battery location was extended. Backup power is now a standard customer request, and the isolation arrangements that make island mode safe are exactly where improvised installations go wrong. Second, the 3rd Edition formally acknowledges PAS 63100:2024, the fire safety specification for domestic battery storage, stating it should be considered, including for battery location.
The PAS 63100 Question: What Actually Binds You
This is where installers get confused, and the chain of reference is worth spelling out. MCS has confirmed that PAS 63100 has no standing in UK regulations and is not directly referenced in MIS 3012, the battery storage installation standard. But MIS 3012 refers to the latest edition of the IET Code of Practice, and the Code of Practice in turn recommends PAS 63100. So the fire safety specification reaches MCS installers indirectly, through recommendation language rather than requirement language. The word is "should", not "shall".
Our advice: treat the recommendation as the requirement. PAS 63100 sets concrete, checkable limits, including a maximum of 20kWh of stored energy per enclosure and installation in accordance with BS 7671, along with location rules that put lofts and escape routes under scrutiny. When something goes wrong, "the standard only said should" is not a sentence anyone wants to say to a loss adjuster. The pattern we see among well-run storage installers is that they design to PAS 63100 by default and document the compliance, which also future-proofs them against MIS 3012 incorporating it directly.
What This Means for Design and Documentation
The practical consequence is that every battery project now carries a design trail: the location decision and its fire safety rationale, the enclosure energy totals, the isolation arrangement for island mode, and the electrical design under BS 7671. None of it is difficult individually; the failure mode is that the decisions get made on site and never written down. Capturing them at the design stage, in the same environment where the string design and single line diagram already live, turns Code of Practice compliance from a memory exercise into an output. That is how Reonic treats storage design: battery placement, electrical validation and documentation in one project record. For the wider picture, see our guides to PV and heat pump compliance and solar PV design software.
Conclusion
The IET Code of Practice is the closest thing UK battery storage has to a technical constitution: not legally binding, but referenced by MIS 3012, aligned with BS 7671 and now pointing at PAS 63100 for fire safety. The market is growing too fast, and battery fires are too visible, for recommendation language to protect anyone. Design to the Code and to PAS 63100, document the decisions, and the paperwork that proves it becomes a competitive asset rather than a chore.
FAQ
Q1: Is the IET Code of Practice for Electrical Energy Storage Systems mandatory?
Not in law, but MIS 3012, the MCS battery storage installation standard, refers to the latest edition of the Code of Practice, which makes it effectively binding for MCS-certified battery work. It is also the benchmark assessors and insurers apply when installations are questioned.
Q2: What is new in the 3rd Edition of the Code of Practice?
Published in April 2024, the 3rd Edition updates the requirements for island mode isolators, extends the guidance on fire safety and battery location, and formally recommends consideration of PAS 63100:2024 for domestic installations.
Q3: Does PAS 63100 apply to MCS battery installations?
Indirectly. PAS 63100 is not law and is not directly referenced in MIS 3012, but MIS 3012 points to the IET Code of Practice, which recommends PAS 63100 should be considered. Best practice is to design to it by default and document compliance.
Q4: What are the key PAS 63100 limits installers should know?
Headline provisions include a maximum of 20kWh of stored battery energy per enclosure, installation in accordance with BS 7671, chemistry-specific standards for battery types, and location rules that restrict placement in lofts and on escape routes for domestic dwellings.
Q5: What documentation should a battery storage project include?
The location decision with its fire safety rationale, enclosure energy totals, island mode isolation arrangement, electrical design and validation under BS 7671, and commissioning records. Keeping these in one project record makes Code of Practice compliance demonstrable years later.
