The last thing California needs is a new barrier to clean energy production. Yet the state Public Utilities Commission is poised to add one at the behest of private companies that see rooftop solar as a threat to future revenue. California’s utility regulator could vote as soon as Thursday on a proposal to undercut the net energy metering program, which has led to rooftop solar installations on more than 1.3 million California households — the most in any state. Under the program, utility companies credit solar customers for the surplus energy they export to the grid, driving down their monthl…