Six months out of the year I try to spend as much time as possible on the roof of my building in Brooklyn, where I'm cooled by a non-air-conditioned breeze, hear birds singing over the street sounds below, and see boats in the distant harbor going by the Statue of Liberty. There's a lot of sunlight up there, even in April and October, not to mention June and July. It's hard not to wonder about solar power. So I was happy to read about the new "New York City Solar Map," which lets you look up most any building in New York and get an estimate of its sun-power potential. But the solar map turns out to be yet another high-concept green proposal that ignores human nature and culture.

Here's the problem: When I find and then click on my own building on the map (which is, of course, way cool—this thing has been done right) I instantly get an estimate that with solar panels our co-op could save $595, generate 2.5 kilowatts of solar power, and reduce our carbon emissions by 2,196 pounds per year. Fabulous, but there's a catch: A little note reminds me that some space must be left clear on the roof, per Fire Department regulations, so that people can reach the fire escape. Then I'm informed how to subtract those few square feet from the estimate.

In other words, the Solar Map assumes that every square foot of the roof (except for the minimum needed for an escape route) will be covered in solar panels.

Remember how I mentioned how much I like to spend time on my roof? I really do. It's like having an extra room—a room where the sun shines and trees sough and birds sing—for nearly half the year. A lot of my neighbors feel the same way: Roofs all around are covered in palettes, decks, balconies, tables, umbrellas, lawn chairs and such like. The decks and picnic tables may reflect the tastes of urbanites who grew up with outdoor space in the suburbs, but the "tar beach" tradition in New York is much older. I doubt if one person in a hundred who enjoys a New York rooftop will be willing to give it up to save a few dollars a month on the electric bill.

Of course, they might not have to. Why not have solar panels cover part of a building roof, and leave room for a couple of chairs elsewhere? It may well be that enjoying a city roof is quite compatible with getting electricity from it. I'm not saying this is an either/or choice.

But the Solar Map project, for the moment, does say that. By ignoring the actual desires and practices of actual people, it has produced estimates that, in thousands of cases, are bound to be wrong.

Photo: A "tar beach" shot from the blog "Transplanted North."