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Mt Tom Solar Array overlooking the Connecticut River

Mt. Tom Energy Storage System

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The Mt. Tom Energy Storage System (ESS) is a large utility-scale battery and the second such system to be installed in the state. The battery draws power, directly from the Mt. Tom Solar Farm, which is the largest community solar project in Massachusetts.

This ESS has been integrated into the Mt. Tom Solar Farm, a clean, cost-effective, renewable power source. Electricity is stored in the ESS isolated from, but interconnected to, HG&E’s electricity grid.  This stored power can be called upon during peak load periods to cost-effectively satisfy demand, while reducing demand-based costs for the utility and its customers.  Demand charges are incurred for electricity and capacity taken from the power grid and used during peak periods.  (In general, the top 1% of peak electric demand hours accounts for 8% of all electricity energy costs.)

HG&E will use this as part of its load reduction program and continue to provide customers with rate stability over the long term.  Additionally, the project helps HG&E’s ongoing efforts to minimize environmental impacts by hosting clean energy production locally.

  • About Mt. Tom ESS
  • Operated by ENGIE Storage (formerly Green Charge Networks)
  • In-service date:  2018
  • Battery Technology: Lithium-Ion
  • Storage capacity: 3 MW/6 MWh-AC Energy Storage System can store 6 MWhs of energy at a rate of up to 3 MW per hour (in or out).
  • Location: Connected to the Mt. Tom Solar Farm, and located on the property
  • Benefits:
    • Keep electric rates stable by reducing rising demand-based charges for HG&E and customers.
    • Increase power quality by maintaining nominal voltage and frequency values – in other words provide grid stability by responding quickly to demand fluctuations
    • Increase use of clean energy sources.
    • Reduce peak load burden on an electrical distribution system, which can extend useful life.


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