It is a widely held belief that leaving lights on saves electricity if one leaves a room for 15 minutes or less. This is due to the energy spike associated with turning on any kind of fluorescent or HID (High-Intensity Discharge) lights. According to the US Department of Energy, turning on a fluorescent light causes an inrush of power that lasts for 1/20th of a second and uses under 5 seconds’ worth of electricity. So this disproves the “15-minute rule” that most people go by, though changing the name would probably create confusion with the other, very important “5-second rule.”
Though flicking off fluorescent lights upon leaving a room (for intervals over 5 seconds) is an energy-saving practice, the energy spike from turning them back on still significantly degrades the lifespan of the bulb. This contributes to CFLs rarely living up to their lifespan claims; they’re not really built to be turned on and off. Thus, it presents another factor to consider when turning your lights on and off: How often do you want to replace your dead light bulbs?
This high-energy spike, thankfully, does not occur in LED lighting. LED lights are solid-state, meaning they are sturdier because they don’t contain delicate, evaporation-prone filaments or gases that ignite with the flick of a switch. This is somewhat apparent in their instant start-up. There’s no warming up period with LED lights – when you turn them on, they just go on, with no additional energy spike. So if you install an LED light bulb, you can turn it on and off without worrying about the “15 minute rule” or the “5 second rule” or any other rules. Now THAT rules.