USA-ENERGY-9

Solar panels are added to the roof of a home being built in San Diego, California August 21, 2015. REUTERS/Mike Blake

A major driver behind the expansion of rooftop solar power in California faces broad calls for reform after researchers concluded the state's "net energy metering" system has benefited the wealthiest ratepayers at other residential customers' expense.

Proposals submitted last week to the California Public Utilities Commission call for restructuring a system that for 25 years has given homeowners with photovoltaic solar panels financial credit for any surplus power they contribute to the state's power grid.