The motion filed Tuesday said the suit, filed by the largest rooftop installation company in Arizona, would usurp the process for setting rates established by the State legislature.

SRP's elected board of directors voted in February to add a new "demand fee" to customers who have installed solar since Dec. 8. The fee is based on the highest 30-minute use of electricity during the month and, combined with other rate changes, will add about $50 a month to solar customers' bills unless they change the way they use power.

Customers who applied to SRP to connect solar panels before Dec. 8 escape the new rate for up to 20 years.

SolarCity Corp. of California, the largest rooftop installation company in the state, filed a lawsuit in response to the rate hike in March.

The suit in U.S. District Court for Arizona accused SRP of "anticompetitive and tortious conduct designed to eliminate solar competition."