Georgia's largest natural gas marketer, Georgia Natural Gas, earned millions more from longtime customers than newer ones over the past 18 months.

Next week is the deadline for customers who want some of that money back.

So-called "legacy" customers have until July 31 to ask GNG for a $25 credit on their bills, based on a pricing scheme that landed the marketer in regulatory hot water this spring.

The $25 credit is part of a March settlement deal between the marketer and the state Public Service Commission, which regulates utilities.

The commission capped the settlement at $2.5 million. If more than 100,000 customers take advantage of the offer, each will get less than $25 when GNG pays the credits in August.

The PSC settlement springs from a change in GNG's variable rate plan in late 2006.

The new plan, the only plan offered to new customers, delivered lower prices than the plan in which existing customers were enrolled.

Both plans were billed as "variable," meaning their prices would fluctuate with the market.

Existing customers could get the new plan if they knew it existed and asked for it.

But the company didn't inform them of the change.

Georgia's second-largest marketer, SCANA, followed GNG's lead in March of last year, and also began offering new customers a cheaper market-rate plan.

Both companies consistently charged existing customers more per therm of gas than they charged under the new plans.

The PSC began getting complaints last year from customers confused that their per-therm rate was higher than the ones posted prominently on the companies' Web sites.

The PSC began investigating. Its staff argued that the companies had violated PSC rules. The companies said they had not, and that the issue was a communication problem only.

GNG settled first.

"We decided to sign the consent agreement with the commission to put this behind us and return to fully focusing on operating our business," GNG spokeswoman Terry Redman said via e-mail. "We continue to believe that we violated no commission rules and broke no state laws."

SCANA reached a similar deal months later, after the PSC probe was finished. SCANA will be issuing its credits in the fall.

In both cases, the new plans were not short-term "teaser" offers. New customers were regularly charged lower "market" rates than existing customers.

At Georgia Natural, the difference ranged from 4 cents per therm to — in one cold month — 22 cents per therm.

In January of this year, pre-2007 variable customers' gas bills were $31 more than new customers' bills, according to estimates compiled by the PSC.

For all of 2007, the long-term GNG customers paid $85 more than new customers, according to PSC estimates. At SCANA, the difference was about $72.

Neither marketer will disclose how many customers were on the older and more expensive variable-rate plans, citing trade secret considerations.

But calculations based on company financial reports at least suggest the impact.

According to the PSC, most gas customers in Georgia buy gas on variable-rate plans, rather than by locking in prices under longer-term contracts.

Assuming that one-third of Georgia Natural's customers had pre-2007 variable plans, the company would have collected $22 million to $30 million extra from those customers in 2007.

For SCANA, using the same criteria, the number would be $17 million.