As more Americans go solar—and save money on their monthly utility bills—electricity providers are doubling down on ways to protect their revenue.

One of the utilities' most widespread strategies is to impose extra charges on customers who are generating their own energy, and they have had varying degrees of success. At least 11 utilities in nine states have attempted this tactic; five have succeeded.

Power providers say these new rates are needed to ensure their customers using solar and other forms of so-called "distributed generation" continue to pay for the basic costs associated with maintaining the grid.

Clean energy advocates fiercely object, calling these efforts "attacks on solar." They argue that the utilities don't adequately account for solar users' benefits to the grid: less electricity is lost during transportation across power lines; less money spent by utilities on infrastructure for transmission and distribution; credits the utilities can potentially use to reach renewable energy goals or tax credits.

Brad Klein, senior attorney at the Environmental Law and Policy Center, closely tracks these rate cases and has intervened in a few. "In all the [rate] cases I've seen so far ... utilities never accounted for solar benefits. You end up with a skewed and lopsided analysis that's insufficient for ratemaking," he said.

The new charges have ranged from an extra $5 per month for the average Arizona Public Service customer to at least $27 per month for typical Wisconsin customers of Rock Energy Cooperative. These fees largely fall into two categories: fixed charges, which remain stable every month, and demand charges, which vary depending on a customer's peak electricity usage.

In certain cases, consumers and environmental activists are pushing back by suing the electricity providers or appealing the rates with state regulators. Their latest win came yesterday, when Minnesota's regulatory commission shot down about $5 worth of monthly fees that Minnesota's People's Electric Cooperative put in place for their handful of distributed generation users.

Klein, who participated in the rate appeal, told InsideClimate News, "I'm pleased the Commission so clearly determined that [People's Electric Cooperative] failed to justify the fee under Minnesota law. It is a clear signal to other utilities that they will need to do a lot more work to be able to justify these kinds of [distributed generation] fees and penalties."

The cost of installing distributed solar at the residential level has declined steadily over the last five years, according to a new report by the Department of Energy's Lawrence Berkeley National Laboratory. In 2014, the median installed price of U.S. residential solar hit a record low of about $4-per-watt compared to more than $12-per-watt in 1998.

Besides rate changes, other hurdles have also been placed in the path of progress for solar, Klein told InsideClimate News. Some states have rolled back solar tax incentives while others forbid customers from leasing solar panels from third-party providers. This "kitchen-sink approach" is occurring in places where there's already high solar penetration such as Arizona, as well as in places with few solar users such as Iowa, he added.

InsideClimate News compiled a comprehensive map of utilities' efforts to tack extra costs onto the monthly electric bills of customers who use rooftop solar panels and other forms of distributed generation.

Correction: A previous version of the story misstated that certain Rock Energy Cooperatives recently received new charges of $90 to their monthly electric bill. This article has been changed to show that these charges added at least $27 per month.