(Reuters) - Nevada’s utility regulator voted unanimously on Friday to require households with solar panels to move to a new, less generous rate structure, angering supporters who said existing solar owners should be able to preserve their previous rates.

The new rates will be phased in over 12 years for all customers with solar panels on their rooftops. The solar industry had lobbied to have existing solar customers “grandfathered” under the current rates for at least 20 years.

The Nevada Public Utilities Commission’s move came in the wake of its decision in December to reduce the compensation solar panel owners receive when they export power they do not use back to their local utility, a policy known as net metering. The December decision sparked intense criticism by solar supporters.

Opponents of net metering, which include many utilities, argue that it shifts the costs of maintaining the grid onto customers who do not have solar panels, even though solar owners also use it.

NV Energy [VNEN.UL], a Nevada utility owned by Warren Buffett’s Berkshire Hathaway Inc, proposed a variety of options for existing solar customers in its public filings, ranging from gradual changes over four years to no changes for 20 years.

The commission “gave the monopoly utility more than it asked for,” said Lauren Randall, public policy manager for solar panel installer SunRun Inc. “This decision is clearly unjust and unacceptable for Nevadans. We will sue to overturn the anti-solar rules, and we will win.”

NV Energy did not immediately respond to a request for comment.

In documents filed earlier this week, the commission’s staff said “the subsidy has become unreasonable,” adding that the 12-year approach would give solar owners ample time to adjust to the new rates.

The December changes to net metering, the solar industry’s most important support policy, prompted SunRun, SolarCity Corp and other installers to abandon operations in the state, saying the new rates meant solar would no longer be affordable to most Nevadans.

Earlier this week, SolarCity blamed the Nevada decision for a weaker-than-expected first-quarter forecast.