ATLANTA — The Southern Co. makes billion-dollar decisions that affect millions of people in Georgia, yet it has attracted little political scrutiny — until now. Leaders of the Atlanta Tea Party are challenging Southern Co. subsidiary Georgia Power over the monopoly’s reluctance to increase its use of solar power, the ballooning costs of building a new nuclear power plant and even its legal right to monopoly status. The group’s action in Georgia seems relatively rare among the loosely linked tea party organizations nationally.