River Mill Storage Tyngsborough, a 500 megawatt battery energy storage facility, is coming to the town of Tyngsboro in Middlesex County, Massachusetts. The developer is Rhynland Energy, a New York based independent power producer assembling a pipeline of standalone storage assets across the Northeast. Documents the company prepared for the Massachusetts Department of Energy Resources describe the Tyngsborough site as a Tesla Megapack 2XL installation with a four hour discharge duration. That configuration lets the facility pull electricity from the regional grid when supply is plentiful and cheap, then push it back when demand and wholesale prices climb. Capacity is the striking number here. A brief filed by intervenors in a separate Energy Facilities Siting Board case states that the Tyngsborough battery site totals 500 MW, of which only 150 MW has been obligated to a state contract, leaving 350 MW uncommitted. Rhynland secured that 150 MW slice through the Commonwealth’s first large scale storage solicitation, held under Section 83E of Massachusetts law. The Department of Energy Resources and the state distribution companies named River Mill Storage as a winner on 19 December 2025 alongside three other projects, for a combined 1,268 MW. Delivery runs through negotiated contracts with Eversource, National Grid and Unitil rather than a purely merchant model, which hands the project a revenue floor before a single Megapack lands on site. Rhynland has not published a construction start date or a capital cost for the Tyngsborough build.

Massachusetts Battery Storage Latest News Puts Tyngsborough in Fast Company

Tyngsborough now sits inside one of the most active storage markets in the United States. Massachusetts is chasing 5 GW of energy storage by 2030, with 3.5 GW of that in the four to ten hour duration band, and the Section 83E round that selected River Mill drew 13 bids against a 1,500 MW target. The most direct comparison is Jupiter Power’s Trimount project in Everett, a 700 MW and 2.8 GWh battery planned for a century old Exxon oil terminal, which pv magazine reported is expected to defer roughly US$2.2 billion in regional transmission upgrades. Flatiron Energy took two further slots with its Energizar project in Somerset and Salt Cod in Chelsea. Immediately south of Tyngsborough, East Point Energy is pursuing a 125 MW and 500 MWh battery in Tewksbury. The Merrimack Valley is quietly becoming a storage corridor. The contrast with the previous generation of Massachusetts assets is stark, as shown by the 34 MW of solar and storage Kearsarge Energy commissioned across 11 sites in Massachusetts and Rhode Island, a portfolio measured in single digit megawatts per location. River Mill alone would carry roughly fifteen times that combined capacity. Rhynland is also learning the political terrain the hard way. Its 180 MW and 720 MWh Moraga project in Oakham won a tentative zoning exemption from the Energy Facilities Siting Board on 25 June 2026 over vocal town opposition, and intervenors in that case argued the uncommitted 350 MW at Tyngsborough could have absorbed the Oakham volume instead.

River Mill Storage Tyngsborough Timeline, Cost and the Permits Ahead

The project’s near term path runs through a permitting regime that changed on 1 July 2026, when the Energy Facilities Siting Board began accepting applications under the consolidated review process created by the 2024 Massachusetts climate law. Storage projects above 100 MWh fall inside that jurisdiction, so River Mill Storage will be assessed under the new framework rather than the older two stage zoning exemption route that Rhynland used in Oakham. Local approvals still matter. Fire safety review, an emergency response plan, conservation commission sign off and site plan approval from Tyngsborough all sit ahead, and the Oakham experience shows how much time community opposition can add. No completion date has been published. The signed contract covers 150 MW, and Rhynland has not said publicly whether the balance of the 500 MW will be built in one phase or staged as later offtake arrives, which is the single largest open question hanging over the state siting process. If the full site is delivered, Tyngsborough would host one of the largest battery installations in New England and give the northern Merrimack Valley a dispatchable resource that reduces reliance on oil and gas peaking plants during summer and winter demand spikes.

Project Fact Sheet

• Project Name: River Mill Storage, Tyngsborough
• Location: Tyngsborough, Middlesex County, Massachusetts, United States
• Project Value: Not publicly disclosed
• Client and Owner: Rhynland Energy
• Total Site Capacity: 500 MW, per an intervenor brief filed with the Massachusetts Energy Facilities Siting Board in April 2026
• Contracted Capacity: 150 MW awarded under the Section 83E Round 1 storage procurement
• Key Components: Tesla Megapack 2XL lithium ion battery units, inverters, transformers and grid interconnection works
• Storage Duration: Four hours, per the company’s filing with the Massachusetts Department of Energy Resources
• Procurement Model: Competitive state solicitation under Section 83E, with contracts negotiated with the Massachusetts distribution companies
• Construction Start: Not yet announced
• Expected Completion: Not yet announced
• Strategic Impact: Contributes to the Massachusetts target of 5 GW of energy storage by 2030 and reduces reliance on fossil fuelled peaking generation

Project Team

• Owner and Developer: Rhynland Energy
• Battery Technology Supplier: Tesla, supplying Megapack 2XL units per the developer’s state filings
• Procuring Authority: Massachusetts Department of Energy Resources
• Contracting Utilities: Eversource, National Grid and Unitil
• State Siting Authority: Massachusetts Energy Facilities Siting Board
• Local Permitting Authority: Town of Tyngsborough
• Main Contractor: Not yet awarded
• Engineering Consultants: Not yet disclosed

Frequently Asked Questions

How big is the River Mill Storage Tyngsborough battery project? The site totals 500 MW according to filings referenced in a Massachusetts Energy Facilities Siting Board proceeding, with 150 MW of that capacity already contracted under the state’s first large scale storage procurement.

Who is building the River Mill Storage Tyngsborough project? Rhynland Energy, a New York based storage developer, owns the project. No main construction contractor has been named publicly, and Tesla is supplying the Megapack 2XL units.

How much does the River Mill Storage Tyngsborough project cost? No capital cost has been disclosed. For scale, the Boston Globe reported the larger 700 MW Trimount battery in Everett at more than US$1 billion.

When will the River Mill Storage Tyngsborough facility be completed? No completion date has been announced. The project must clear the Massachusetts consolidated siting process that opened to applications on 1 July 2026, plus local fire safety, conservation and site plan approvals.

Why is Massachusetts building the River Mill Storage Tyngsborough battery? The state is targeting 5 GW of energy storage by 2030 to firm up renewable output and cut the use of expensive oil and gas peaking plants during demand spikes.