MadSci Network: Engineering

Re: Does turning flourescent lights on/off use more electricity?

Date: Fri Jun 4 19:31:08 1999
Posted By: Barry Kamrass, Faculty, Electronic Engineering, the engineering consortium, inc.
Area of science: Engineering
ID: 928151274.Eg

Excellent question!  You have to look at it in two parts:
a:  the cost of the electricity alone
b:  the overall life cycle cost of the light that you're paying for.

a:  There's no question that turning the lights off immediately after 
leaving the room saves electricity.  How many $ that saves depends on how 
much you're paying for electricity.  But that's not the whole picture.

b.  When you cycle a fluorescent light on/off, you shorten the life of the 
bulb, starter, ballast. These cost money to replace, lost productivity 
while the fixture has failed, and labor costs to replace the bad parts. The 
numbers I've read vary, but it seems that if you're only going to leave the 
light off for anywhere from 30--90 minutes, depending on who you listen to, 
you're better off in total life cycle cost if you leave the light on.

One company I worked at had the fluorescent lights turned on by infrared 
motion sensors in the room, and would leave the lights on for 45 minutes 
when the motion sensor was activated.  This seems like a good idea, and 
obviously the company's calculations said that 45 minutes was the optimum 
number in terms of total life cycle cost.  So your idea is validated in at 
least one real instance.

One small point: a common spelling error is to spell it "flourescent".
The correct spelling is "fluorescent".

One caution:  if you're at a party with engineers or science students and 
you mention this 
subject, an argument will invariably result.  Of course, that might be 
interesting--your call.  Good Luck!

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