An environmental group and a conservative foundation have filed separate lawsuits in Fulton County Superior Court challenging the law.
Gov. Sonny Perdue signed the measure into law in April. The utility says that allowing it to raise rates beginning in 2011 will shave roughly $300 million of the Plant Vogtle expansion's $14 billion price tag.
The Southern Alliance for Clean Energy said in its lawsuit that it forces ratepayers to pick up the tab for "an untested technology with extraordinary costs."
And the Fulton County Taxpayers Foundation said the rate hike amounts to an unconstitutional "tariff."