The Massachusetts Electric Co. which now operates under the Grid umbrella argued that four difficult storms lowered its rating in a state review of its 2006 performance. The company asked that the storms be excluded from the review.
DPU said in the ruling it would not exempt the storms because Mass. Electric had fallen short in the rest of its performance review.
National Grid spokeswoman Debbie Drew said the company was disappointed, but is looking for the best way to credit the $8 million to its customers.
The utility must also rebate $30,000 to customers of Nantucket Electric, also a National Grid company.