Congress, in its wisdom, overrode them with one national standard in which windows are rated on both their ability to keep heat in and their ability to keep sunshine out. That works fine for, say, the mid-Atlantic, where about the same amount of energy is used for summer cooling and winter heating. But in colder climes the sun's heat is needed for warmth--meaning New Englanders will have to pump up their thermostat if they want a tax-credit window that does a good job at keeping the sun out. In warm states residents don't need windows that excel at trapping heat. "You don't want the same window in the far South and the far North," says Lowell Ungar, director of policy with the Alliance to Save Energy.