Grant had an inductive current loop, a computer-based digital sampling oscilloscope, and a question that most households have quietly argued about for decades: is it actually cheaper to leave the lights on than to keep flipping the switch? The answer turns out to hinge on a number so small it is almost insulting. For anyone who has ever walked out of a room mid-argument about the electricity bill, the data settles it fast.
The startup surge that turned out to be nothing
The experiment started with six types of light bulb, each measured for how much energy it draws during normal operation. The second half of the test was where things got interesting: Grant rigged an inductive current loop to a digital sampling oscilloscope capable of capturing what happens in the millisecond range, right at the moment a bulb switches on. After running several trials, a clear pattern appeared. There was always a spike at startup as the filament warmed up, but it flattened out to steady state remarkably fast.
With the raw data in hand, Grant ran the calculations for each bulb type. The incandescent result stopped the conversation cold. Total power used during startup came to 21,522 watt seconds. Total power for one second of continuous use came to 59,519 watt seconds. The break-even time, the point at which leaving the light on would have cost the same as the startup surge, worked out to 0.36 seconds.
Grant put it plainly: ‘That means I’m leaving the room to go make a cup of coffee. It’s only worth it for me economically to keep the light on if I come back in a third of a second.’
The other five bulbs followed the same shape. The compact fluorescent clocked in at 0.15 seconds, faster than a blink. Halogen landed at 51 seconds. The standard fluorescent tube drew the most startup energy of the group, equivalent to 23.3 seconds of continuous use, but even that number collapsed under real-world scrutiny. Unless a room is packed with fluorescent tubes and the occupant plans to return in under 30 seconds, the math does not support leaving them on.
Whether switching the light off wears out the bulb faster
The verdict was unanimous: the myth that leaving lights on saves money is busted.
The LED Era: The myth is busted, but the savings have vanished
- The Old Penalty: Leaving a single incandescent bulb running 24/7 used to cost a household a noticeable amount of money over a year.
- The Modern Cost: Running a single modern LED bulb continuously for 24 hours draws so little power that it costs roughly 4 cents.
At the end of the longevity test, the LED sat in its socket, still lit, long after every other bulb in the group had given out.
