“Leaving a light on in a room uses more energy than turning it off and on again.”

The idea that leaving a light on consumes less energy than turning it off and on again is a persistent misunderstanding rooted in the technology of older lighting. This belief largely originated with incandescent light bulbs, which were the standard for many decades. When an incandescent bulb is first switched on, its cold filament has lower electrical resistance, causing a brief surge of current, known as inrush current. This momentary spike of power, lasting only a fraction of a second, led some to believe that the energy expended during startup outweighed the savings from turning off the light.

However, scientific evidence reveals that this startup surge is negligible in most practical scenarios. For incandescent bulbs, the energy used in that brief inrush is equivalent to only a few seconds of the bulb's normal operation. Given that incandescent bulbs are highly inefficient, converting only about 5-10% of their energy into visible light and the rest into heat, turning them off consistently saves far more energy than the minimal startup cost. Older fluorescent lights, particularly compact fluorescent lamps (CFLs), also had a startup surge that was slightly more significant, and frequent switching could reduce their lifespan. For these, the general recommendation was to turn them off if you were leaving a room for more than about 15 minutes.

The misconception persists because people often overestimate the duration and magnitude of the startup energy spike, or they might recall anecdotal advice from a time when lighting technology was different. The visible flicker or slight delay in older fluorescent lights when first switched on might have also contributed to the idea that a significant amount of energy was being "wasted" during startup. With modern LED lighting, this concern is virtually non-existent. LED bulbs are incredibly efficient, and turning them on and off has no notable impact on their energy consumption or lifespan. Therefore, for almost all lighting types, the most effective way to save energy is simply to turn off the lights when they are not needed.