There's an old debate as to whether it's better to leave a fluorescent light on for long periods time, rather than turn it on and off as you leave and re-enter a room. The old wisdom is that the energy used to get the light started is more than is needed to leave it on. Is this correct?
-- Mark and Meredith, NSW
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Nowadays CFLs (Compact Fluorescent Lamps) are one of the most energy efficient bulbs, and they don't have the problems of humming and flickering associated with the old-style fluorescent bulbs which usually came in tube shapes. This is because CFLs use an electronic ballast, rather than the magnetic ones found in the old fluorescent lights.
When CFLs are running they are quite energy efficient (using just 20 per cent of the energy used by incandescent globes), but it is true that the energy levels spike when the light is first turned on.However, this spike only lasts for 1/10th of second.
Basically, the amount of energy in the spike equates to the amount of energy the bulb would use during 5 seconds of normal operation. So the only way that you would use more energy than normal, is if you turn your fluorescent lamp off and on more frequently than every 5 seconds!
When CFLs are turned on, a tiny bit of an electrode coating is burned off. Once this coating runs out, the light will die. However, the bulbs usually have enough coating to last more than 6,600 start-ups. That means if you turn your light switch on and off continually, you are using up the electrode coating faster than normal, and therefore shortening the lifespan of the lamp. However, if you leave your lights on constantly, you are paying for the operating costs of the light.
What it comes down to is a trade off between electricity bills on one hand, and the price of new lights and the maintenance needed to change them on the other (and of course the environmental consideration of saving energy and resources!).
Turning lights on and off essentially only uses the equivalent of 5 seconds extra electricity. The only disadvantage is that the bulb will wear out a little faster. Nowadays CFLs are reliable and inexpensive, so it is best to be energy efficient and switch off your lights when you don't need them.