The failure of a bill to preserve incentives for homeowners with solar panels sets the stage for Maine regulators to implement their own ratched-down version of so-called net metering.

Last winter, the Maine Public Utilities Commission voted to establish a new set of rules that gradually phases out incentives for rooftop solar installations. The rule is set to begin Jan.1, but conservation groups are preparing to challenge it in court.

Currently, solar homeowners who sell their excess power to the grid are compensated with a credit at the full retail rate of that electricity, an arrangement called net metering. The PUC rules, expected to be implemented in January, gradually reduce those rates.

Specifically, the PUC rule would grandfather all existing net-meter customers, and any who install solar before Jan. 1, 2018, for 15 years. Their incentives would stay as they are today.

New customers who install solar over the next 10 years would have the credit on the transmission and distribution portion of the electric bill decreased at a rate of 10 percent each year. For instance: After Jan., 1, 2018, net-solar customers who send excess power to the grid would be credited for 90 percent of its retail value on the transmission and distribution portion of the bill, each year, for 15 years. They would still get a full credit on the supply portion of the bill.

Customers who install rooftop solar in 2019 would get an 80 percent credit, and so on.

The reasoning behind this formula, the PUC said, was to try to match financial incentives with the expected pace of falling equipment prices, thereby maintaining a similar payback on investment for homeowners.

In Central Maine Power’s service area, a typical home with a 5-kilowatt solar array that can generate all of its power demand on an annual basis would earn a credit worth roughly $35 a month, or $420 a year, according to ReVision Energy, the state’s largest solar installer.

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