A new audit finds national grid overcharged upstate electric and gas customers $8.5 million in 2009.
Downstate customers may have been overcharged $35 million in gas usage around the same period.
Overland Consulting looked at 1425 of the company's transactions over a 20-month period, from October 2008 through May 2010.
In that time, Overland found errors in about 176 transactions, or 12.4 percent.
National Grid, meanwhile, says Overland's findings are "deeply flawed."
It acknowledges it made mistakes, but says its records show only $175,000 in overcharges, or about .02 in errors.
National Grid says it takes issue with the methodology Overland used to arrive at its findings, saying the consulting firm only looked at a fraction of the company's transactions.
In Albany on Thursday, the Public Service Commission voted unanimously to begin the procedure to figure out whether National Grid owes refunds to its customers.
As it takes steps to do that, National Grid is working on an evaluation of the auditor's recommendations for improvement.
The company has 45 days to submit a plan to the PSC, outlining how it will improve its internal accounting system.
National Grid says it's already taken measures to improve since the 2009 audit.
