Turn Me Off or On?...It All Depends on How Long

Q: Is it better to leave fluorescent bulbs on continually or turn them on and off as needed? Is there a time when it is more efficient to leave them off?

Sincerely,

In the Dark

A: Dear Dark Side:

Your question of efficiency is a good one. I would qualify the question by asking, is it better to leave fluorescent bulbs on continually or turn them on and off as needed as compared to what? If you are comparing a fluorescent bulb to an incandescent bulb than the answer would be yes in either situation. It would be more efficient to have fluorescent bulbs period.

So what is the difference between the two? A incandescent light bulb uses a thin tungsten filament that electricity runs through. Since the filament is thin it offers resistance to the electricity. This resistance turns electrical energy into heat. The heat turns the filament white hot which emits light. It also emits heat. This is highly inefficient. In fact only 10-15% of electricity that incandescent lights consume results in actual visible light.

A fluorescent bulb utilizes electrodes at both ends of a fluorescent tube. A gas mixture of argon and mercury vapor is pumped into the tube. When the light is switched on a stream of electrons flow from one electrode to the other passing through the mixed gas. The electrons bump into the mercury atoms and activate them. As the frenzied mercury atom slows its activity it emits off ultraviolet photons. The photons collide with phosphor that coats the inside fluorescent tube. This collision creates visible light. This process is 4-6 times more efficient than incandescent bulbs and produces less heat.

If your question is in comparison with the fluorescent bulb itself the question still needs to be qualified. How long will you be leaving a room? This is what makes all the difference. The process to emit light from a fluorescent bulb does take more energy upon the switch of a light as compared to leaving the light on. So wouldn't it be more efficient to just leave it on? Just like everything else in life the efficiencies of a fluorescent bulb have a break even point. So yes, there are times when it is more efficient to leave a fluorescent bulb but when doesn't seem to be agreed upon. Provided are recommendations as to when to turn the bulb off.

• US Department of Energy: If you leave the room for more than 15 minutes
• Consumer Energy Center: When leaving the room for more than 3-5 minutes
• Myth Busters: When leaving the room for more than 23.3 seconds
• Lighting Design Lab: When leaving a room for more than 15-20 minutes

But consider this:
According to the Arogonne National Laboratory, the energy needed to start a fluorescent lamp is not significantly higher than the energy needed to operate it. Even if we assume that a fluorescent lamp uses twice the normal amount of power during the starting phase, this phase lasts for only about 1 or 2 seconds. So, the crossover time to save energy would be less than 2 seconds.

And finally Scientific American recommends to always turn them off.

What you are really failing to ask yourself is the most important question regarding fluorescent bulbs. That question is, How does one make an authentic Star Wars light saber using fluorescent bulbs? Well, you can't unless you want to kill yourself or end up like those brits who put gasoline in fluorescent lights. You can try something a bit Safer however.