The costs of solar energy are plummeting, and now are about on par with the electricity generated at big power plants. This new reality intensifies a long-running business and regulatory battle, between the mainline electric utility companies and newer firms that provide solar systems for homeowners' rooftops. Sometimes the rivalry looks more like hardball politics than marketplace economics.

The way rooftop solar typically works, the homeowner leases rooftop panels from a company that owns and installs them. It can be an expensive proposition, but the homeowner saves some money by drawing less power from the utility company's electric plants, and even by selling some solar power back up the electrical grid to the utility.

Utilities say rooftop solar users need to pay their fair share to maintain that grid.