MARBURG, Germany — This fairy-tale town is stuck in the middle of a utopian struggle over renewable energy. The town council’s decision to require solar-heating panels has thrown Marburg into a vehement debate over the boundaries of ecological good citizenship and led opponents to charge that their genteel town has turned into a “green dictatorship.”

Old and new coexist in Marburg, where a hilltop castle overlooks a solar-powered building. The city seeks to expand solar use. Some Marburg residents are concerned about how pending solar rules will affect historic buildings like these in the city center.

The town council took the significant step in June of moving from merely encouraging citizens to install solar panels to making them an obligation. The ordinance, the first of its kind in Germany, will require solar panels not only on new buildings, which fewer people oppose, but also on existing homes that undergo renovations or get new heating systems or roof repairs.

To give the regulation teeth, a fine of 1,000 euros, about $1,500, awaits those who do not comply.

Critics howled that the rule, which is to go into effect on Oct. 1, constituted an attack on the rights of property owners. The regional government in Giessen stepped in and warned that it would overturn the rule.

City officials in Marburg said, in turn, that they would take their case either to administrative court or all the way to the Hessian state capital, where they would try to get the state building code changed to protect their ordinance from officials in Giessen.

In the middle of this political chess match sit homeowners like Götz Schönherr.

From his deck, Mr. Schönherr can see the town’s famous hilltop Gothic castle as well as two of its three power-generating windmills. On his roof, a solar panel glints in the sunlight. He already uses the solar energy to heat his water, which has allowed him to turn off his boiler for roughly six months a year, a boon for his pocketbook but a decision he said he made for the sake of the environment.

And yet Mr. Schönherr opposes the new ordinance.

Mr. Schönherr had hoped to reinsulate his home, but to do so, and to satisfy the solar regulation, he would have to install a larger solar panel. It would cost him close to $8,000.

“That leads, in my case, and I would think in other cases as well, that people say, ‘Well, let’s just not reinsulate the roof,’ ” Mr. Schönherr said. “So it’s absolutely counterproductive.”

Officials in Giessen agree. “We have no problem with the use of solar energy,” said Manfred Kersten, press spokesman for the regional government in Giessen, “but this was a poorly constructed ordinance.”

Germany is one of the world’s top champions of reducing greenhouse gas emissions and promoting renewable energy. Thanks to hefty federal subsidies, the country is by far the largest market for photovoltaic systems, which convert sunlight into electricity.

Marburg, a historic university town where the Brothers Grimm once studied, is a model of enlightened energy production and consumption. In addition to the windmills and solar installations, the town’s utility company buys hydroelectric power from Austria, is transitioning its fleet of buses and other vehicles to natural gas and even lights footpaths with solar-powered lamps. As a result, the Marburg dispute sometimes feels like an argument between the enlightened environmentalists and the really enlightened environmentalists.

“Marburg is already a leader when it comes to the use of solar energy, but up until now they’ve always tried to convince people rather than forcing them,” said Hermann Uchtmann, the opposition politician behind the “green dictatorship” charge who leads a local citizens political group, the Marburger Bürgerliste.

Like Mr. Schönherr, who is a member of the group, Mr. Uchtmann hardly fits the predictable mold of the Luddite opponent of renewable energy. He is a chemist at the local university who once built a solar-powered desalinization station for the town’s sister city, Sfax, Tunisia.

“It’s unfortunate that they decided to compel people, because I think you breed opponents that way rather than friends of solar energy,” Mr. Uchtmann said. He said he found the demands too invasive for existing homes, especially in the case of older citizens who might not live long enough to justify the upfront costs of installing the solar systems.

“I’m right up against the border myself,” said Mr. Uchtmann, who is 64. But he said he could support a solar-heating requirement for new buildings.

Because the town of 80,000 has a level population and relatively few new homes are built here, restricting the measure to new construction would not go far enough for the politicians behind it.

“We have a serious energy problem with the older homes,” Marburg’s deputy mayor, Franz Kahle, said in an interview at the historic town hall on the city’s colorful market square. To make a real leap forward, he said, a dramatic step was necessary.

“Before, solar installations were the exception and their absence was the rule,” Mr. Kahle said. “We want to get to the point where the opposite is the case.”

He pointed out that building codes constantly dictated what property owners could and could not do with their homes and said that the solar regulation already offered exceptions for cases of hardship or alternatives for those living in the shadiest spots.

Marburg’s law has attracted attention nationwide as a model for environmentally active politicians.

“What they are doing in Marburg is good and progressive, and we, and other cities, need to move forward with similar initiatives as well,” said Birgit Simon, deputy mayor of Offenbach am Main and a member of the Green Party . She said she hoped a coalition of left-of-center parties in the state Parliament could change the building codes to make the Marburg ordinance sustainable and imitable.

Among Marburgers interviewed one sunny afternoon this week, there was near universal support for the ordinance’s goals but an almost equal level of confusion about its exact nature.

“In principle, it’s a really good idea,” said Cornelia Janus, 35, who works at the university. But she questioned whether the costs might be too high and whether historic buildings and monuments would be protected.

“For a city like Marburg,” she said, gazing toward the churches and the castle arrayed along the hillside, which draw tourists from around the world, “that’s pretty important too.”