A Long Island senior citizen just discovered that she had been overpaying her National Grid bill for a decade.

It turns out she never told the utility she converted from oil to gas. Lawmakers say more customers could have made the same, costly mistake.

"I thought it was outrageous, the bill I got"

The 92-year-old, who did not want to be identified, is now taking charge of her National Grid bill, and says she won't be fooled again.

"I thought it was outrageous, the bill I got," she said. "We just look at the sum and we will pay the bill."

She said she has taught herself all the nuances of National Grid codes and charges.

"The code I was on was 140 with National Grid, but my bill said 120," she said.

That means she was being billed at a higher gas appliance rate than a space heating rate, when her home went from oil to gas.

Her son, Christopher, who lives in Texas, said she switched from oil to gas 11 years ago.

"My mother has kept every single bill," Christopher said.

"There could be thousands of customers who are overpaying"

The family recently contacted National Grid to put on pressure. The woman's son said the utility treated them with respect, but he claims, "There could be thousands of customers who are overpaying."

Suffolk County Legislator Robert Trotta said Mario Mattera, a state senator from the woman's district, is pursuing legislation that could protect ratepayers in such a situation.

"God knows how many other people are paying the wrong rate for their gas," Trotta said.

National Grid is not required to refund overpayment due to incorrect rate codes, but in this case said it will work with the family.

"We understood the customer's concerns and are working with them toward an equitable solution. Our automated review had already updated the rate before they reached out, and we regret any frustration this may have caused. Each year, we proactively review accounts to help ensure customers are on the most appropriate rate, even if they haven't contacted us," National Grid said in a statement.

The 92-year-old said her situation should be a learning experience for others.

"We are looking at other things now, the codes. People should be more conscious," she said.