Regulators have approved an electricity discount for Amazon on central Ohio, a plan that will increase in size as the online giant adds data centers.

But many specifics of the discount -- such as an estimate of the dollar value and the percentage savings -- have been not been disclosed publicly because Amazon says the information is a trade secret.

The Public Utilities Commission of Ohio voted 5-0 on Jan. 10 to approve the special electricity rate, which was jointly proposed by Amazon and American Electric Power, the region's main electricity utility.

"It is important to note that although the construction of this arrangement exempts, limits or discounts Amazon's payments of some bill items, ultimately, consumers' bills will not increase as a result of the commission's approval today," said Asim Haque, the PUCO chairman, following the vote.

He referred to the fact that no additional line item would appear on customers' bills as a result of the plan.

However, consumer advocates have said that customers' costs will rise indirectly, because AEP will be getting less money from Amazon, leaving others pay more to cover costs of AEP programs and service.

Amazon applied for the discount through a subsidiary company called Vadata, which builds data-center complexes.

The plan is tied to Amazon's announcement that it wants to build up to a dozen additional data centers in central Ohio. It already has three complexes in central Ohio -- one each in Dublin, Hilliard and New Albany.

Haque said the plan recognizes that data centers are not typical businesses.

"Data centers are enormous consumers, enormous enough that if Amazon completes construction of their cloud-based facilities, they will be significantly larger than any other customer in AEP Ohio's footprint," he said. "The arrangement also represents what could be a new era for economic development in the state."