Ask A Scientist© Environmental Science Archive

Author:     williamh
Text:       Is it true that it is better to leave a fluorescent light on 
rather than turning it on or off over a short period of time? If this is true, 
does the rule hold true for all kinds of fluorescent lights? What is the 
threshold point in terms of time for leaving fluorescent lights on or turning 
them off? A secretary in my building is concerned about energy conscious 
workers who continually turn the fluorescent lights to the photocopying room 
on and off. Thanks for answering these questions.

Response #: 1 of 2
Author:     Don Libby
Text:       Yes, most of the energy used by any kind of fluorescent light is 
used up in getting the gas excited enough to glow (which happens the instant 
you turn it on) the rest of the time the bulb is just coasting. Sort of like a 
jet plane taking off - it takes almost no fuel at all (or much less) to keep 
it at cruising altitude than to get it there in the first place. Energy-
conscious workers who flip fluorescents on and off all day are big energy 
wasters though I do not know what the break-even point is as far as leaving 
them on for extended periods. Try your local electric utility company.

Response #: 2 of 2
Author:     Don Libby
Text:       If you do not mind hearsay, here is a rule of thumb that I go by: 
if a light will be on for less than 1/2 hour, incandescent bulbs are more 
efficient. If a fluorescent light is left on for less than 2 hours, it is 
being used inefficiently. Turning a fluorescent on twice within two hours uses 
more energy than turning it on once and leaving it on for two hours. Flipping 
fluorescents on and off all day also causes them to wear out more quickly, so 
it causes a solid waste pollution problem as well (i.e. throwing away worn out 
bulbs and ballasts and replacing them with new one not only fills up the dump 
with burned out bulbs, but increases the amount of waste generated in the 
process of manufacturing new bulbs).