A building that runs on half the usual amount of energy? How ho-hum. These days, most new construction in Philadelphia can do that without even trying, simply by adhering to the U.S. Green Building Council's basic LEED standards. What would be really interesting is if someone put up a major building that consumed no energy at all.

Hold onto your electric bills, folks, because the first net-zero apartment house in the United States is coming to a most unlikely spot: a forlorn lot on Kelly Drive in East Falls that has languished for more than a decade. Equally astonishing is that it's being built by a first-rate local design firm, Onion Flats, under the auspices of that most ossified of city agencies, the Philadelphia Redevelopment Authority.

Just 30 American projects have been certified so far by the Passive House Institute, the organization that sets U.S. standards for net-zero buildings - and all but four are single-family homes. To qualify as passive, a building must produce nearly as much energy as it draws from the power grid, relying on a mix of technology that includes solar panels, heat exchangers, and extreme levels of insulation.

The result is net-zero or, more accurately, near-zero consumption. In passive-energy circles, the joke is that you should be able to heat your house with a hair dryer.

It's one thing for a custom-designed private house with a generous budget to accomplish that feat, but not so easy to get the same result with an exponentially larger, profit-driven, multifamily project. Onion Flats' seductive design, which revolves around a lush interior courtyard, promises 130 apartments, a cafe, shops, a covered garage - but no heating furnace. No one in the United States has attempted a passive-energy project this big, although several have been completed in Europe.

"We understand this is not without risk," concedes Ed Covington, who took over the Redevelopment Authority in 2010 with the aim of energizing the city's plodding land bank. He hopes the ambitious project, called Ridge Flats, will work on the agency like a can of Red Bull.

Of course, a particularly Philadelphia-style hurdle lies ahead. To get the quality construction and watertight seals that passive-energy projects require, Onion Flats plans to fabricate the project in modular sections in a Pottstown factory, then truck them to the site for assembly. A seamless envelope that keeps out winter drafts and summer heat is the secret weapon of passive houses. Walls at the Ridge will be almost 12 inches, twice the normal thickness.

But off-site fabrication is not likely to go down well with Philadelphia's notoriously territorial trade unions. Onion Flats has successfully used its modular construction system, called Blox, for several small projects, but nothing with the visibility of this Schuylkill site, just steps from the Falls Bridge.

The Redevelopment Authority (which recently changed its initials from RDA to PRA) isn't the only one that has a lot riding on the success of Ridge Flats. For all the talk about containing our nation's carbon footprint, the passive-house concept has remained the poor stepchild of the more comprehensive LEED rating system, which ranks buildings on a wide variety of environmentally friendly features.

Some sustainability experts question whether the costs justify the effort. Passive-house standards were established in Europe, where energy prices are higher. Many prefer LEED because it encourages designers to think about the big picture of sustainability, not just energy costs. Still, the success of a big, sexy urban project such as Ridge Flats could demonstrate that near-zero buildings aren't just for people who don't have to watch the thermostat.

Ridge Flats also marks a leap in scale for Onion Flats, the freewheeling Kensington design firm led by Howard Steinberg and the three McDonald brothers - Tim, Pat, and John. Although they are developers as well as architects, their projects rarely exceed a dozen units. They've repeatedly lost competitions to more established and conventional developers.

Ridge Flats gives them a stage as big as their ambitions. Tim McDonald sees it as a test site for virtually all the firm's future work because it allows them to combine the latest passive-energy technology with low-cost, modular construction. "This is where the world is going," he insists.

Onion Flats owes this opportunity to the cultural change at the Redevelopment Authority. Three other developers competed for the project, including local powerhouses Pennrose Properties and Westrum Development Co.

They offered the usual faux rowhouse architecture, but the agency wanted more for the prominent site, Covington says. Once owned by the Rivage banquet hall, the parcel fell into the PRA's hands in 1998 thanks to then-Councilman Michael Nutter, who feared the land would be gobbled up by a gas station.

Covington says Onion Flats' $25 million proposal was a standout by every measure: looks, sustainability, and public access. In the past, the authority would have simply given the project to the highest bidder, as the Philadelphia Industrial Development Corp. did when it recently sold off a parcel at Broad and South Streets.

Onion Flats' proposal combines a fresh, modern design with equally fresh thinking about modern social life. It has oriented the project to take maximum advantage of its position at the head of the popular Kelly Drive recreation loop. For years, runners and cyclists have turned around at the Falls Bridge without a decent spot to enjoy a coffee. Ridge Flats rolls out a welcome mat with a space for a terrace cafe. You know Onion Flats is right for the job because it remembered to include a generous row of bike racks on the rendering.

The cafe terrace and shops along Calumet Street and Ridge Avenue will help shield a grade-level parking garage. The plan calls for four stories of housing on the garage podium. Those units will be arrayed around a green roof garden.

Onion Flats has a talent for finding a double purpose for every design move. For instance, the hallways will be located on the exterior of the building so the space doesn't have to be heated and cooled. The placement ensures the apartments will have windows on two sides, for cross ventilation. Onion Flats also includes small nooks along the walkways to serve as porches, increasing the chances for social interaction.

Ridge Flats won't break ground until early in 2013, but in the meantime Onion Flats will perfect its passive-house skills building two other projects: a trio of affordable houses in North Philadelphia and nine luxury rowhouses called the Stables in Northern Liberties.

It's often said that hard times produce innovation. The Empire State Building went up in the depths of the Great Depression. A project like Ridge Flats just might show that saving energy isn't so hard to do.