When I was a kid, my dad followed behind me, shutting off the incandescent lights I left burning around the house. “You’re wasting energy,” he’d scold as I tried to slip out of the room. He was right, of course. In the 1980s, 5 to 10 percent of an average household’s electricity bill went to keeping the lights on. So when my own son was born last June, my dad joked he was waiting for the day when his grandson would exact his revenge on my utility bill.