The state's highest court has reduced penalties imposed against two electric utilities by Massachusetts regulators for their poor responses to a pair of 2011 storms that left hundreds of thousands of customers without power for days.

The Department of Public Utilities fined National Grid $18.7 million for its response to the outages caused by Tropical Storm Irene in August 2011 and by a snowstorm in October. NStar was fined $4.1 million.

The Supreme Judicial Court on Thursday cut NStar's penalty in half, saying the agency did not prove that the utility failed to respond in a timely way to downed power lines after the storms.

The justices reduced National Grid's penalty by $900,000.

The court did not change a $2 million penalty against Western Massachusetts Electric Co.