Perhaps no issue pits urbanists against themselves more than the conundrum of solar panels in historic districts. Civic-minded liberals are supposed to be both for renewable energy and preserving architectural heritage and older, typically denser, and more pedestrian-friendly, housing stock. But when a homeowner wants to put a solar panel on his roof, those values may come into conflict, with local bloggers taking sides (usually pro-solar panel).

The tension between preserving architectural landmarks and increasing energy conservation has existed for decades. According to the National Trust for Historic Preservation, it first began with homeowners installing new double-glazed windows, but it has become more common in recent years as home-based green energy production becomes more widespread.

Solar panels are the first energy-creation technology to enjoy mass market penetration, as they have recently become sufficiently cheap for more homeowners to install arrays. New companies such as SolarCity offer homeowners the opportunity to receive a free installation and split the savings on their energy bill, in essence renting the panels for no money down. As a result, home solar panel installation is up dramatically: 76 percent in 2012 from the year before, according to a report by Solar Energy Industries Association and GTM Research, while the average cost of a solar system declined by 27 percent.

This has inevitably led to conflicts with historic preservation in landmarked buildings or neighborhoods. The historic preservation rules are less stringent for buildings that merely contribute to a neighborhood’s historic character than for buildings that are landmarked themselves. In historic districts, there is often no specific guidance on solar panels, leaving preservation commissions scrambling on how to evaluate proposals. Often they simply say the rules are the same as for any new roof fixture.

But roof fixture standards also vary by locality. In Washington, D.C. the fixture must not be “prominently visible” from the street. That’s more or less the norm in many towns and cities. But how does one define visible? In Glen Ridge, N.J., rooftop solar panels have been rejected when looking barely detectable in photographs. Even within a city, some historic districts are more heavily policed than others. In D.C.’s tony Georgetown neighborhood, solar panels must pass inspection by a neighborhood commission as well as a citywide agency.

When solar panel installations go before historic preservation commissions in older East Coast cities--where historic districts are mostly filled with flat-roofed row houses--they are typically approved, although sometimes through negotiation and modification. Solar panels on flat roofs are usually invisible from the street now that technology allows the arrays to lie flat instead of standing upright. Historic row houses often have parapets that block the view of the roof, and when necessary they can sometimes be extended or added on to protect solar panels from the street view.

So, the conflict between landmarks and solar collectors typically arises in historic communities with suburban-style houses that are detached and have slanted roofs. In Washington, D.C., a proposed solar panel array on the front slope of such a house’s roof in the city's Cleveland Park historic district was rejected in 2012 by D.C.’s Historic Preservation Review Board. The building's owner contended that because his house faced south, it was uneconomical to install solar panels unless they could be on his roof's front slope, from which it was inevitably quite visible, albeit low-profile in the sense that it was black and laid flat. The D.C. government says it offered compromises to place solar panels elsewhere on his property.

In New York City a new fixture must be “appropriate,” which is defined as something that, “does not profoundly impact the historical appearance of the building.” That may sound more restrictive, but because it does allow for visibility, it may actually be less so. New York, because it has so many historic tall buildings, long ago came to terms with modern mechanical fixtures on roofs. “Without water towers, Spiderman wouldn’t be able to get around,” jokes Simeon Bankoff, executive director of the Historic Districts Council. “We’ve seen a lot of mechanical innovations happen to us. We’ve seen [solar panels] go to a public hearing a couple times and never have they [the Landmarks Preservation Commission] denied it outright. More often than not they say, 'OK, if you push it back towards the back of the building or raise the parapet wall a bit.'”

Preservation must sometimes also contend laws that encourage homeowners to adopt renewable energy features. In California, the 1978 Solar Rights Act prohibits unreasonable restrictions on solar energy systems. That would seem to tie preservationists’ hands, but they say that owners thinking of installing solar panels often come to their local commission wanting to preserve their community’s character. (They also do not want to lose their historic status, which is necessary to obtain state tax credits for rehabilitating historic properties.) California preservationists have become adept at walking homeowners through alternatives to rooftop solar panels. Energy audits, for example, often reveal that comparable energy savings can be achieved through more modest measures such as re-caulking windows. “You can buy shares on a solar farm to get the same tax credit that you’d get from putting solar panels on your roof,” notes Milford Wayne Donaldson, FAIA, chair of the Federal Advisory Council on Historic Preservation and former California State Historic Preservation Officer. “We’ve looked at alternative installations, like putting collectors on the ground instead of the roof, adopting solar panels that look like shingles, solar panels that look like skylights that would be historically appropriate. I think now people can get the best of both worlds.”

Sometimes historic preservation rules are modified to account for renewable energy features, as in St. Louis, which has loosened requirements that new rooftop installations be invisible. Solar panels will be approved if they are "visually compatible" and the Preservation Board may also approve panels that must face the street to obtain efficiency when they determine that "all efforts have been made to minimize the visual presence of the installation."A lot of communities are looking at exceptions for solar panels,” says Stephen Calcott, deputy preservation officer for the Washington, D.C. Office of Planning, about what he found in looking at towns and cities across the country to help determine how D.C. would approach the issue. “In some communities they’re establishing more specific guidelines for solar panels or sustainability: essentially an acknowledgment by those communities that issues related to sustainability needed their own standards. Other communities are saying ‘They’re like any other type of alteration and they need to be discreet.’ It’s not that they’re not going to be approved, but they need to follow the guidelines of any other roof addition or alteration.”

Preservationists themselves are split as to what is the right approach. “If we could get every commission to update their guidelines to provide some guidance to homeowners on renewable energy installations that would be fantastic,” says Kimberly Kooles, who worked for the National Trust for Historic Preservation on integrating renewable energy into historic properties.

But some preservationists argue that we should not create special loopholes for solar panels when technology is constantly changing. And they fear the precedent that could be set for other renewable energy installations, such as windmills, that could be far more unsightly than solar panels. As soon as installing wind turbines on your roof becomes economical, the conflicts will begin anew.