Depends on type of controller. There's basically 3 ways to run lighting. Your standard timer, where all lights turn on and off simultaneously. Then theres a flip box, which powers your ballasts 24 hrs a day, and every 12 hrs the power is switched from room A to room B. Then the newest type that i'm aware of is called "zig zag" relaying, and thats where someone would have say 20 lights in one room. But instead of all the lights burning for the whole time, you'll have groups of lights that'll burn for say 4 hrs, then it'll switch to another group of lights. Somebody came upon the realization that the sun doesn't burn full intensity the WHOLE day, EVERY day. So switching groups of lights on and off like that will allow you to cover more area with less power. But that's obviously for growers on a slightly larger scale.