strange_complex: (TT Baby Helios)
I've been wondering recently about how energy-efficient it is to switch off a light when you know you're going to switch it back on in only a few minutes' time (e.g. because you are only popping out of the room to go to the loo), and also how much the situation varies depending on what type of light you are using.

Googling produced a few results, which is a start, although they don't seem entirely authoritative to me. So I'm reporting back on my findings here, a) in case anyone else has been wondering about the same issue, and b) in the hope that someone can point me towards more detailed and convincing information on the subject.

According to this page, the initial surge in an ordinary incandescent household light bulb "would probably burn up one-tenth of a second's worth of regular electric light burning - or maybe a second, absolute maximum". So in other words, you may as well turn off an ordinary bulb even if you know you'll be switching it back on in five minutes' time.

Regarding fluorescent lights (which are what I have in my office at work), The University of Alberta has apparently recently been trying to encourage its staff to save energy by turning off unused lights. They say, "Turning fluor(e)scent lights off and on causes some wear. Studies show that if you turn them off and on in periods of less than 20 minutes it reduces the beneficial effects. Incandescent lights - more common in homes and on desktops -- can be switched off even if the space is vacant for less than 20 minutes."

This, of course, is a slightly different issue - they are really talking about the life-time of the fluorescent tubes, not how much energy they use while being switched on as compared to while running normally. But I guess they must take up a fair amount of energy when they are produced in the first place, so conserving the tubes themselves is worth thinking about - although I would really like to know how the two things play off against one another. And of course the University of Alberta offer no link or reference to whatever 'studies' they are talking about.

Meanwhile, on the same issue the University of Virginia's 'How Things Work' page reckons by a much shorter time-frame: "Since turning an incandescent bulb on and off doesn't shorten the life of its filament significantly, you do well to turn it off whenever possible. The same isn't true of a fluorescent tube--turning it on ages its filaments significantly (due to sputtering processes) so you shouldn't turn a fluorescent lamp off if you plan to restart it in less than about 1 minute."

One minute? Twenty minutes? Those are quite different lengths of time! I guess either way my office lights are probably best left on while I'm nipping out to the loo or to pick something up from the staff room. But, as I say, it would be handy to have a more authoritative guide than this.

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