Conscientious people turn off the lights when they’re the last person to leave a room. I imagine that back when incandescent lighting was king, that probably made a lot of sense - leaving the lights on when nobody’s around would be wasting a substantial amount of money through both electricity usage and bulbs burning out. But, if the light being turned off is a modestly sized LED one, and especially if an electrician would be hired to replace a worn out light switch, I wonder if the situation is so clear-cut?

Each time the switch is turned Off, a small cost is incurred due to the wear and tear on both the switch and the light. If the switch is left On, the cost incurred is a function of time; the light continues to consume electricity, and the light has a finite running lifetime - this is the case that seems to be more commonly considered.

So, the question becomes: how long does it take until the cost of leaving the light On exceeds the cost of turning the light Off. It only makes financial sense to turn the light Off if it’ll stay that way for longer than the break even time.

I’ve also wondered how hard it would be to insert a javascript calculator in to a blog post (answer: not very). What a nice coincidence! Perhaps there’ll be more of these in the future, maybe with graphs!

Power draw of lights Watts
Cost of electricity $/(kW*hr)
Replacement cost of light $
Rated hours of light hour
Rated on/off cycles of light switches
Replacement cost of switch $
Lifetime of switch switches
Break even off time


So, we see that for an LED light, the break even time is indeed about 10 times longer than for an equivalent (in terms of light output) incandescent - the light I’m using in the example is apparently equivalent to a 90W incandescent bulb, those typically have a lifetime of about 1000 hours. In typical day-to-day light switching consideration, this all shows that it probably does make sense to continue turning off lights when you’re the last person out the door.

Notes:

  • There is a slight simplification in the above - it doesn’t account for the fact that when a light fails due to on/off cycles, the new bulb won’t have any hours on the clock (or vice-versa).

  • In a few minutes of searching, I couldn’t find an on/off cycle rating for a typical light switch. The light specifications come from a box for an LED light I have handy.