The candy machine at Henry Sibley High School knows when students roam the halls and automatically powers down when they’ve gone home. The basketball court still shines, but under the glow of fluorescent tubes that suck up a fraction of the juice the old lights used. Thanks to such measures, energy costs across the school district in this Twin Cities suburb already are down by nearly a third. Officials want to trim the expenses more, but that will require investment in upgrades. The federal economic stimulus dollars could be just what they need. Some of the billions of dollars trickling down from Washington will be used to make public buildings energy efficient.