As the distributed energy landscape shifts from simple generation to more complex storage and energy management, codes and standards are evolving in tandem. The publication of the 2026 NEC (National Electrical Code), the 2026 NFPA 855 (Standard for the Installation of Stationary Energy Storage Systems), and the 3rd Edition of UL 9540 (Energy Storage Systems and Equipment) has provided new requirements as well as clarifications that are essential for manufacturers, system designers, and installers to understand.
We will explore these changes in a two-part series: the first focuses on ESS-related updates to the National Electrical Code; the second covers NFPA 855 and UL 9540.
When Does a Battery Become an Energy Storage Systems (ESS)?
The dividing line between traditional stationary batteries and ESS has been a bit hazy over the last few cycles of the NEC, forcing the industry to navigate a gray area between Article 480 Stationary Batteries and Article 706 Energy Storage Systems. The 2026 code cycle sharpens the distinction, bringing the differences into focus.
Historically, Article 480 provided requirements for electrical installation of all batteries. Article 706 first appeared in the 2017 NEC, but came with overlapping requirements and created confusion regarding where the scope of one Article ended and the other began. The 2026 NEC takes a decisive step to clear this confusion. The scope of Article 480 (Batteries) covers applications like uninterruptible power supplies (UPS), individual batteries that are not part of a listed ESS, and battery chemistries that aren’t required to be listed. Article 706 (ESS) applies to engineered systems listed to UL 9540 with components including batteries, inverter, controls, and protection that are evaluated as a single unit (note not every ESS includes an inverter).
| NEC Article | Applications | Listings |
|---|---|---|
| Article 706 | Energy storage systems >1kWh including lithium-ion, capacitors, and flywheels | Complete system listing to UL 9540 |
| Article 480 | Stationary batteries >1kWh not covered by Article 706 | Equipment and batteries must be listed, except for vented lead-acid and nickel cadmium batteries |
| Other Articles | Other dedicated battery uses and mobile applications | Addressed in articles specific to the application, such as 517, 624, and 625 |
The word “standby” was removed from the title of Article 480 and elsewhere in the article in the 2026 cycle, with the rationale that standby is a usage description which may be applied to either stationary batteries or ESS. Furthermore, the definition of “stationary standby battery” was deleted entirely, leaving just the definition for “battery.” This improved language paves the way for a distinction that is primarily listing-based: requirements for an ESS listed to UL 9540 are found in Article 706, whereas other stationary battery applications (i.e. not a listed ESS) are still governed by Article 480.
Refining the Definition of ESS
All NEC definitions are found in Article 100. A revision of the Energy Storage System definition now aligns it directly with NFPA 855 Standard for the Installation of Stationary Energy Storage Systems. An ESS is defined as “one or more devices, assembled together, capable of storing energy to supply electrical energy at a future time.” The plan is for the NEC and the NFPA 855 definitions to remain aligned in future cycles.
Crucially, this definition implies that an ESS is evaluated as a complete system, including protection, controls, power conversion equipment, and warning systems. The definition of ESS is technology-agnostic—covering electrochemical batteries, capacitors, flywheels, and potentially compressed gas—and the accompanying informational note points directly at UL 9540 as the relevant standard.
Listing Requirements and Reconditioning
The Article 706 listing requirement has been moved in alignment with code-wide structural updates. Previously located in 706.5, the mandate that ESS be listed is now found in 706.2. Additionally, all references to UL 9540 were removed from Article 480.
A new addition to the 2026 NEC is Section 706.3, regarding reconditioned equipment. The code now explicitly prohibits the installation of reconditioned energy storage equipment. The Article 100 definition of “reconditioned” involves restoring equipment to operating conditions, distinct from normal servicing or one-to-one replacement. The substantiation for this prohibition notes that currently there is a lack of data and requirements to support the safe reconditioning of these complex systems. This restriction applies to a full ESS and may change in future code cycles; furthermore specific components within an ESS might still be eligible for reconditioning under Section 110.20, the general allowance for reconditioned equipment (which applies unless prohibited elsewhere in the NEC).
Commissioning and Residential Applications
Section 706.7 has been expanded. In the 2023 code, commissioning requirements exempted one- and two-family dwellings. For 2026, commissioning is now required for all installations, including residential homes, and must be performed in accordance with manufacturer instructions. Furthermore, servicing requirements have been clarified: all ESS must be maintained in safe and proper operating condition. While residential systems are still exempt from the requirement to keep written service records, the mandate for maintenance is now universal (as it should be in our opinion).
Disconnecting Means and Safety
The requirements for disconnecting means in Section 706.15 have been harmonized with Article 705 (Interconnected Electric Power Production Sources). Section 705.20 contains requirements for source disconnecting means, and is written to provide overarching rules, with ESS-specific additions in Article 706. The requirement in 2023 Section 706.15(D), which called for a disconnect within sight of the ESS if circuits passed through a wall or partition (and applied to both the ESS input and output), has been deleted. However, 706.31(E) still requires that overcurrent protection be provided at the energy storage component end of the circuit if ESS input or output circuits pass through a wall, floor, or ceiling.
An emergency shutdown function to cease export of ESS power (not explicitly required to be a disconnect) is mandated for one- and two-family dwellings per 706.15(B). This has been in the NEC for several cycles but the language has been revised. Note the emergency shutdown initiation device must be located at a readily accessible location outside the building.
Circuit Calculations and Rounding
A subtle but safety-critical change occurred in 706.30(A) regarding the maximum rated current for inverter input circuits. Previously, the language implied a calculation was necessary (rated power divided by lowest input voltage). This calculation often yielded a number lower than the manufacturer’s rated continuous input current, potentially leading to undersized conductors. The 2026 NEC clarifies that the manufacturer’s rated continuous input current should be used. Additionally, rounding is now explicitly permitted for these calculations: results can be rounded to the nearest whole ampere, dropping decimals smaller than 0.5.
Flywheels in the House? Not Yet…
Remember not every ESS is lithium ion or even a chemical battery! Part VI of Article 706 regarding other technologies has a change related to flywheels (FESS). Previously banned entirely for residential use, flywheels are now only prohibited inside the dwelling structure. They may be installed outdoors or in detached structures for one- and two-family dwellings, in compliance with installation requirements found in NFPA 855.
What questions remain?
The 2026 NEC is indicative of a maturing energy storage industry, moving away from treating batteries as generic components and acknowledging them as sophisticated, integrated devices that have specific, system-level hazards and safety considerations. The NEC cannot address these considerations alone; ensuring safe installation and operation also requires robust product safety standards and detailed fire and life safety requirements. This is the subject of Part 2 of this article: LINK