For years, utility executives have watched the rapid spread of rooftop solar power with trepidation, seeing a potential threat to their profits.

A new federally funded report suggests that it’s not an idle fear.

Rooftop solar could cut utility earnings by as much as 15 percent, according to the report from Lawrence Berkeley National Laboratory. It could also raise electricity rates for utility customers, but by a far smaller amount, up to 2.7 percent.

The report, issued Wednesday, will stoke the already heated debate over solar power policies in California, Colorado and other states. Utilities fear a future in which more and more customers generate their own power, leaving others to shoulder the costs of paying for the electrical grid. The scenario even has a name, the “utility death spiral.” In California and elsewhere, the companies have responded by pushing to end or change “net metering” policies that pay solar homeowners for excess electricity they ship onto the grid — a major incentive to install PV panels.

Lawrence Berkeley researchers decided to study the financial effects of customer-owned solar on utilities, their shareholders and their customers. To do so, they created two model utility companies, one located in the Southwest, the other in the Northeast. Since not all utility companies operate in the same way, they decided that the hypothetical Southwestern utility would own some of its own power plants while the Northeastern one wouldn’t.

The researchers looked at what would happen to utility costs and revenues as the use of solar power grew, rising to 2.5 percent of the utilities’ total retail sales, then to 10 percent. (For comparison, rooftop solar generation in Pacific Gas and Electric Co.’s territory in 2013 equaled 2.3 percent of retail sales, according to the report. PG&E has more solar customers than any other U.S. utility.)

The Southwestern utility fared better than its Northeastern counterpart. But each took a hit to its bottom line.

At 2.5 percent solar penetration, the Southwestern utility’s earnings fell 4 percent. At 10 percent solar penetration, profits dropped 8 percent. For the model Northeastern utility, the profit cut started at 4 percent and increased to 15 percent.

The utilities’ hypothetical customers also felt a pinch, as the costs of grid maintenance were spread across a smaller base of sales. But the effect was far more modest. At the low end of solar growth, average customer electricity rates rose .1 percent for the Southwestern utility and .2 percent for its Northeastern counterpart. At the high end of solar penetration, rates climbed 2.5 percent in the Southwest and 2.7 percent in the Northeast.

So what can utilities do? The researchers offered several options, although each has its own pros and cons. One intriguing possibility, already suggested by former U.S. Energy Secretary Steven Chu and others, would be for utilities to jump into the solar business themselves and start leasing panels to their customers.