SACRAMENTO, Calif.—California regulators proposed major changes to the state’s booming residential solar industry Monday, including reducing the discounts homeowners with rooftop solar and storage systems get on their electric bills when they sell extra energy back to the power companies. California’s successful program to get more people to put solar panels on their homes has been at the center of a fierce debate between the state’s major utilities and the solar industry, and the California Public Utilities Commission’s proposed reforms have been highly anticipated. The state’s three major utilities—Pacific Gas & Electric, San Diego Gas & Electric, and Southern California Edison—say the savings solar customers get now are so great that those customers no longer pay their fair share for the operation of the overall energy grid. The CPUC’s proposal would reduce the incentives for going solar and roughly double, to 10 years, how long it takes Californians to make …