A state board in California has approved a proposal to require solar panels on all new homes beginning in 2020, a measure that would increase the cost of new construction but provide savings on utilities — and help the state meet ambitious targets for reducing greenhouse gas emissions. California, which is routinely a leader in environmental regulatory efforts, would be the first state in the country with such a requirement. Several cities, including San Francisco and South Miami, Fla., have residential solar panel requirements. The new rule in California would cover all low-rise residential buildings, although houses that are frequently...