Concord light plant staff flag safety, transformer limits in debate over letting home batteries push power onto grid

Jason, the light plant director, told the Concord Light Board on July 8 that residential battery storage technology has advanced faster than some regulations and that allowing batteries to "push back" power onto the grid could create safety and equipment risks.

The director summarized regulatory and technical constraints: Under a 2019 DPU interpretation, a battery that charges from both solar and the grid cannot participate in net metering tariffs, and national standards from Underwriters Laboratories and IEEE include anti-islanding protections and other safety requirements. "Battery storage as a technology has evolved a lot faster than the regulations," Jason said, and he noted the IEEE requirement that inverters detect unintentional islanding and stop backfeeding within about two seconds.

Why it matters: staff said export-capable batteries can create fast, concentrated flows of power that were not anticipated when many local transformers were sized. Staff described a typical interconnection practice that assumes a worst-case solar export when sizing transformers and warned that retroactively allowing export could require time-consuming, disruptive transformer upgrades. "If we suddenly allowed customer number 3 to then push back after we had already done the interconnection for the solar and battery, that transformer might not have the capacity to do that," Jason said.

Board members and staff also discussed operational controls and program pathways. Staff described software and interconnection controls that can limit export, but cautioned that manufacturer or installer changes can alter settings later. "We do a witness test when we leave," Jason said, describing one safeguard; Joe Repoff added that the town requires a roughly 300-second delay before distributed systems reconnect after a restoration to reduce transient risks.

Program context: Laura Scott, who leads the rebate/process team, gave program metrics: "Right now, we have a total of 37 systems either online or pending interconnection," she said, listing total inverter capacity at about 485 kilowatts and total storage of 1,044 kilowatt-hours (just over 1 megawatt) as of July 1. Staff noted average residential systems in Concord are roughly 15 to 20 kilowatt-hours each.

Alternatives and pilots: staff pointed to existing programs such as the Clean Energy Group's "connected solutions" and the locally operated "connected homes" program (through MWEC and a platform such as Virtual Peaker) that allow utilities to dispatch enrolled devices during limited peak events. Those programs, staff said, can permit controlled discharge to the grid where allowed by tariff design. Jason told the board the recently passed vehicle-to-grid tariff has been filed with regulators and that the pilot could provide insight into larger-scale discharges.

Public comment and next steps: a resident, Pamela Drit of 13 Concord Green, urged the board to pursue export and larger-scale local storage to build virtual power plants and argued that compensating solar customers supports cheaper electricity for the whole community. The board did not vote on policy changes; staff recommended continued study, analysis of transformer impacts and watching pilot results before changing the ban on pushback.

The board asked staff to continue studying options, gather comparative data from communities that allow export, and return with suggested guardrails or a pilot design; no formal policy change was enacted at the meeting.