Ameren Illinois says solar capacity has reached the threshold where state law says net metering should be phased out.
Consumers Energy failed to develop data and facts to justify slashing solar credits almost in half, challengers say.
Iowa utilities and clean energy groups both supported a new net metering law but now disagree on its interpretation.
Solar advocates are concerned that the transition to a localized value-of-solar rate is happening too soon.
A proposal, which still needs to pass in the legislature, would preserve net metering and let utilities recoup costs faster.