That’s the opinion of David Crane, chief executive officer of NRG Energy, a wholesale power company based in Princeton, N.J. What’s afoot is a confluence of green energy and computer technology, deregulation, cheap natural gas, and political pressure that, as Crane starkly frames it, poses “a mortal threat to the existing utility system.” He says that in about the time it has taken cell phones to supplant land lines in most U.S. homes, the grid will become increasingly irrelevant as customers move toward decentralized homegrown green energy.