A proposed rate hike by Xcel Energy for homes with solar energy panels is sending a wave of concern through the state’s solar energy installation industry.

Xcel estimates that for an average Boulder home with a 4.5-kilowatt solar array, the annual charge would be about $23.

But the Colorado Solar Energy Industries Association calculated that for very efficient solar homes, the fee could add up to $200 a year or more.

The charge will dampen the sale of solar energy systems, said Beth Hart, director of the solar industries association.

“No one is going to buy a system open to an escalating charge their neighbors aren’t paying,” Hart said.

Since March 2006, about 3,000 homes and businesses have installed 33 megawatts of solar power valued at about $200 million, according to the industry association.

“There is some tension between Xcel and solar power users,” said Jeremy Nichols, energy director for WildEarth Guardians, an environmental group. “To some degree they are in competition.”

The proposal for a base monthly charge on homes that “net meter” or sell into the Xcel grid as well as purchase power is to help pay for the cost of transmission lines, according to Xcel officials.

A charge for transmission and distribution lines is built into the rates for the average customer, said Tom Henley, an Xcel spokesman.

Homes with solar that net meter and use far less power haven’t paid that charge.

“It’s only fair everyone pay for the system,” Henley said, adding that the electric grid provides a “backup” for the solar homes.

For example, that Boulder home with a peak month of 879 kilowatt-hours would have a minimum monthly rate of $22.87, according to an Xcel filing with the Public Utilities Commission.

In months where the house used more electricity than the minimum rate, the bill would be higher. In months where electricity use dropped because of solar offsets, the homeowner would still have to pay that $22.87 minimum rate.

Xcel estimates the total annual cost to such a Boulder homeowner for power that wasn’t actually used would be $22.75.

But Xcel concedes the rate could be higher.

The industry association says, for example, that a home that used 879 kilowatt-hours for one month and used no electricity for the other 11 months would have a minimum monthly rate of $22.87, equal to an annual charge of $274.

Mark Jaffe: 303-954-1912 or mjaffe@denverpost.com

How the charge would be set

Under the proposal, homes with solar systems installed after April 2010 would pay a minimum transmission charge based on the peak month’s electricity consumption in a year.