Power consumed by switching on a light

I seem to recall hearing that there is an amount of power used in the process of switching on a light. That is, my lit-up 100W light bulb is using 0.1kWh each hour, but there was some extra power used in order to turn it on in the first place.

Am I remembering this correctly? (i.e. is it true?)

Is there an easy way to estimate how much power is used in this way?

Is there any effect on this based on how recently the light was turned off? That is, if I flick the light off and on fairly quickly, am I wasting a bunch (relatively) of power?

Basically, I’d like to know: if I’m going out, and then coming home to cook dinner, should I turn off the kitchen lights?

I tried Googling this but was unsuccessful.

Incandescent lamps have a large in-rush current. That means that they consume much more power when they are heating up than they do when operating. That said, the inrush time is extremely short - something on the order of 1/20 of a second, so the real issue is not power usage, but lamp lifetime. Turning on a lamp shortens it’s life considerably. A rule of thumb is to leave it on if you are going to use it again in 1/2 hour.

Old florescent lamps use more current when the light starts. Sodium lamps like on street lamps use more power at start up. They use more power until they warm up too. I think that is true for mercury vapor lamps too. In the 70’s they were pushing for energy conservation, one thing being to shut off the lights. This is when the news and such started saying you waste energy by turning off the lights for certain types. Over time it gets repeated and changed until the bit about only certain lights do this is lost. I’m sure others can add to the list. Incandescent bulbs you should just turn on and off. I don’t know how the new compact fluorescents stand, because they use a higher frequency and contain solid state components. The higher frequency and solid state design should use less start up current than the old transformer 60 htz ones.

I can guarantee that HID lamps use the least power at startup (ignoring the short inrush current), and slowly reach full power after 10-15 minutes or so. I’m reasonably sure that all discharge lamps work this way.

Mythbuster did this, and they determined the power used at startup was fairly insignificant. I can’t say anything about their testing procedure.

The sodium lamps consume power the whole time as they power up or warm up. Until they warm up they put out much less light. You have to turn them on for many minutes before they light the room enough to actually see where you’re going.

The Encyclopedia of Alternative Energy and Sustainable Living

Thanks! Good old Mythbusters :slight_smile:

True, but that is not the issue.
It’s power consumption we’re talking about.

The power consumption per lumen of light is higher, is what I’m saying. If you don’t think the warm up time influences if you should turn it on or not fine, but I think it does. There were multiple questions asked.

Go back and look at your statement in my post #4.

The cutoff times for different lamps looks one way when you consider power consumption, and it looks another way when you consider out of pocket costs that include power and lamp replacement. The fluorescents I just installed downstairs say their components are balanced so that if they are run in sessions of a half hour the two most perishable things in the tube should run out at about the same time.

Coincidentally, I calculated this very thing sometime last year. I don’t have time to reproduce the calculation at the moment, but I do remember that the break-even point for incandescent bulbs was a fairly small fraction of second (maybe of order one tenth?). That is, if you were leaving for, say, thirty milliseconds, then you might leave the lights on.

For what it’s worth, I have met multiple people who were under the mistaken impression that tuning a normal lightbulb off and then on would use up electricity equivalent to leaving it on for an extended period of time.