Although the efficiency of that is very low, the power is stored when demand is off peak, when supply is in excess and demand is low resulting in that power being low priced. Because of the low efficiency, only a small fraction of that power is later returned to the grid, but at a much higher price during peak load periods when New England's electricity consumers place the heaviest demand on the system.
Although the energy storage has very low effectiveness, it stores power when electricity demand is minimal and supply exceeds it, resulting in lower prices. However, due to its inefficiency, only a small portion of this stored power is returned to the grid during peak hours when New England's consumers place the greatest demand on the system.