Mesa staff propose zoning and fire-code standards for battery energy storage systems

City of Mesa staff on Oct. 6 presented draft zoning and fire-code amendments intended to regulate battery energy storage systems (BESS), a land use staff described as "emerging and increasingly prominent" and carrying unique public-safety and compatibility concerns.

"BES is an electrochemical device that charges and collects energy from the grid or a generation facility," Planning Director Mary Kopaski Brown said. She told council the draft ordinance would create a land-use definition for BESS and distinguish principal from accessory uses based on nameplate capacity: principal if greater than 1,000 kilowatts (1 megawatt); accessory if below that threshold and serving only the host site.

Staff proposed allowing principal BESS facilities in General Industrial (GI) and Heavy Industrial (HI) districts subject to approval of a planned area development (PAD) overlay, giving council discretion similar to the approach used for data centers. Accessory BESS would be allowed in agricultural, residential, commercial, employment and downtown districts, staff said.

The draft includes several site- and safety-related standards. Appiah, development services director, summarized technical provisions the fire and building divisions are pursuing: 10 feet between individual battery units; a minimum 100-foot setback from the property line to the first battery; and a 400-foot separation from the property line to "sensitive receptors" such as residential zoning, schools, parks or churches, which together can mean roughly 500 feet from the nearest battery to the nearest sensitive receptor. The fire-code proposal would also require breaks in arrays longer than 300 feet, 26-foot access lanes for arrays over 150 feet in length or depth, and other access standards to accommodate firefighting equipment.

Under the draft, BESS applicants would submit an operational plan with a compliance statement, a preliminary emergency-response plan (outlining shutdown, de-energizing or isolation procedures), a decommissioning plan for end-of-life site cleanup, and an augmentation plan describing periodic battery replacement. Kopaski Brown said the augmentation and decommissioning plans will be enforced through the fire code but must be submitted during zoning review so the fire department can evaluate safety considerations early in project design.

Staff also would require an initial sound study to establish ambient decibel levels at the nearest sensitive receptor and annual sound testing for five years. "We're asking them to not increase that ambient sound level," Kopaski Brown said; if levels rise above the baseline, operators would be required to implement mitigation measures demonstrated by an acoustic engineer.

Council members asked about financial assurances to cover decommissioning if an operator abandons a site. Appiah said the city is still evaluating options, including security bonds, and will study what other jurisdictions have done before recommending a specific approach. "Those are the things that we're still exploring," she said.

Vice Mayor Summers and staff urged that the city track and adopt evolving national standards; Summers recommended NFPA 855 and noted UL 9540A testing standards are being updated, and staff said they plan to incorporate the most current building and fire codes as they are adopted.

Staff said at least one BESS project — a facility on Signal View/Signal Butte Road — is already under construction and that the draft rules aim to address both new proposals and the community impacts of existing projects. Staff also said they have conducted initial public meetings and industry outreach and will hold a virtual public meeting on Oct. 13, 2025, from 6 to 7 p.m. to solicit more feedback. A formal ordinance with PAD requirements and specific technical standards is expected to return to council later in the year for formal consideration.

No final adoption took place at the Oct. 6 study session; the presentation was informational and staff indicated standards may change based on public input and updated national testing and fire-safety standards.

Practical details summarized from staff proposals: nameplate capacity threshold of 1,000 kW for principal use; 100-foot internal property setback to first battery; 400-foot separation from property line to sensitive receptors (roughly 500 feet from battery to sensitive receptor when combined with internal setback); 10-foot spacing between battery units; 150-foot separation from commercial/industrial buildings; initial and annual sound studies (baseline and five years); augmentation and decommissioning plans; and PAD-required approvals for principal facilities in GI and HI zoning districts.

Next steps: staff will continue public outreach (virtual meeting 10/13) and incorporate national standards and jurisdictional research before returning a draft ordinance to council for formal action.