Nova Scotia Power says it has overcharged about 13,000 customers on their latest bills.

The problem is the efficiency rate, which was charged twice to the bills in question.

“What happened this year in 2014 was that the timing of the efficiency charge increase was separate from the general rate increase. So because of the glitch that we had on the system, unfortunately, because the timing of the two increases were separate it calculated the efficiency increase twice for that billing cycle,” Nova Scotia Power spokeswoman Neera Ritcey.

She said customers who were overcharged will be notified on their next bill and reimbursed.

“We are going to be doing the adjustment on the bill and letting them know through their next bill that this error had occurred,” Ritcey said.

The utility apologizes, she added, and is working to make sure it doesn't happen again.