A new idea is coming to light in Massachusetts, both locally and statewide — making solar panels mandatory on new construction.

The city of Watertown passed a new ordinance in November that requires solar photo voltaic (PV) panels on most new commercial buildings. Despite agreement that the policy is an important step forward for reducing the emissions that cause climate change, Watertown’s policy has received pushback from some commercial developers and builders, calling into question how energy code changes should be made in Massachusetts.

But the ordinance also has had another effect — other communities are beginning to consider similar proposals, as are state lawmakers.

Less than 10 percent of Massachusetts' energy generation comes from the sun. A statewide building code went into effect last year that requires all commercial buildings to be designed to accommodate solar panels. Watertown Energy Manager Ed Lewis says that in Watertown, developers of commercial buildings would design buildings to accommodate solar panels but wouldn’t actually install the panels.

"More often than we'd like, they'd come in with the solar assessment where it looked like a good plan, looked like a good solar array, and they're like, 'Yeah, we're not doing it.' That was very frustrating," Lewis said.

Lewis recently pointed to the dozens of solar panels spread across the roof of the Watertown police station as the type of thing Watertown is trying to encourage.

“Beautiful southern exposure," he said. "We're up high here so that we've got a great, great opportunity to make a ton of electricity from the sun."

It's the kind of solar array he’d like to see on more buildings, hence the city’s ordinance.

"Bottom line, if you're building a commercial building in Watertown, and you're over 10,000 square feet, and you have a good solar orientation, you got to have 50 percent of your roof area covered with solar PV," Lewis said.

While many commercial developers agree that adding solar panels is generally a good thing, they don't necessarily like being forced to do it. For one thing, roof space can be in high demand.

"With climate change, many owners are moving their HVAC systems up to the roof, and that takes up significant rooftop area,” said Tamara Small, CEO of NAIOP Massachusetts, the commercial real estate development association. “If this proposal would take effect, I don't know where you'd end up putting … that equipment. In addition, many of our developers are thinking about green roofs to address stormwater issues in light of increased storms. That wouldn't be allowed under this proposal."

Lewis said that's not necessarily true. The Watertown ordinance allows the town flexibility to evaluate projects on a case-by-case basis, he said.

"Watertown is not going to force anyone to put something up that doesn't make sense," he said.

Even so, Small said the Watertown ordinance might be against state law, which prevents individual communities from adopting their own energy or building codes.

"We have a statewide energy code for a reason,” she said. “We need to have one uniform energy code."

Small said a patchwork system of different requirements in each community would make it difficult and expensive for commercial developers to work.

Massachusetts, she pointed out, has a state board that's responsible for the statewide building code.

"The board was created to oversee these types of requests and deny them unless they were very specific and special situations, which this just does not seem to fulfill in any way," she said.

But Lewis said Watertown’s ordinance is covered by the state's so-called Stretch Energy Code, which allows cities and towns to have more aggressive energy requirements than are in the statewide building code. And, he said, he hopes the policy does lead to statewide change.

"Somebody has got to be the first one,” Lewis said. “I'm glad it was us. This is the type of ordinance that I would love to see go statewide."

In fact, bills introduced in the state House and Senate would do just that — and more. The state bills require solar panels on new commercial buildings and on new single-family homes.

"We believe if we took this step, within about 25 years we would double the state's solar capacity,” said Rep. Michael Connolly, who introduced the House bill.

Like the Watertown ordinance, Connolly said there would be exemptions from the solar requirement if it wouldn't work or isn't affordable in certain locations. And there are programs where homeowners can get solar panels by essentially leasing out their roofs to companies that put up the initial investment.

But homebuilders like Michael McDowell said buyers should have a choice about what’s on their new roofs, and he worries about the cost.

"Any time you have a huge cost increase to any part of the building code, you're knocking potential people out of the building market," McDowell said.

McDowell is on the Massachusetts Board of Building Regulations and Standards, which makes changes to the statewide building code, and he said that’s the process that should be used for proposing changes like this — not bills in the state legislature or local ordinances. They’re working on the 10th edition of the code now.

“We could easily incorporate it there, if it had merit,” he said.

Supporters of the state bills and local ordinances say the idea does, indeed, have merit, and it’s a key step toward stemming climate change.