My dad, who’s an electrical engineer, once told me that fluorescent lights use less energy when left on for 24 hours than it takes to start one up every time you turn it on. Is this true? And if so, is it also true for these newer-fangled fluorescent “bulbs”?
No, it’s not true, and never has been. There is a small penalty for starting the lamp, but it’s measured in minutes or seconds.
However, the lamp lifetime is reduced by frequent starts, so leaving them on might increase the lifetime slightly. These days, it’s better turn turn the light off if you are not going to use it for more than 5 minutes or so.
As an EE, your father should have been able to figure this out for himself - how much energy does a fluorescent tube take - 40W? If you run it all day, that’s 960Whr. A tube starts up in seconds, which would imply that it was drawing many thousands of Watts during that time, which clearly isn’t the case.
This link suggests 15-20 minutes between starts.
Even incandescent bulbs have in-rush currents. Not sure what the “payback time” is for an incandescent. But I would suspect it’s less than a fluorescent bulb’s payback time (which, as stated above, is already very low).
Lots of electrical loads have in-rush currents. 
They looked at this on “Mythbusters”, and found that while fl. lamps have a extremely brief (microseconds) high current draw on start up, that it was more economical to turn them off even when not needed for less than a minute.
FML
Thanks. My dad told me this, if he did, like 40 years ago; I may well have misremembered it. Thanks for clarifying.
It was never an energy issue, it was a maintenance issue.
Back in the old days, flurescent tubes had separate ballasts. The theory (and I have no idea whether it was true) was that turning the fixtures on and off caused the ballasts to wear out and need to be replaced before the filaments. The cost of the electricity needed to let the light burn continuously was less than the cost of the janitor needed to figure out whether the ballast or the bulb had burned out, and swap out a burned out ballast.
However, except in 24-hour a day locales (like a late-night cleaning crew) or for security reasons, it was still standard practice for the last one out to turn off the lights.
Myth busters tested modern bulbs. The idea of not turning lights on and off goes WAAAAYYYYY back when they had monster ballasts. Can’t remember what the rule of thumb was but it was nowhere near 24 hrs. Can’t remember the last time I saw an industrial fluorescent fixture like the ones in the 60’s which were old looking when I was young.
Back 40 years ago, the ballasts would wear out from constant on/off and they were hwaaaay more expensive and hard to change than the bulbs. Thus in some cases, some businesses when figuring the manpower needed, and the then relatively cheap power, decided that it was cheaper just to leave them on.
So your Dad is out of date, and it wasn’t the power so much as the expense, but he’s more or less right.
Ah, yes, I remember this from my aquarium installation days; you had to make sure there was little shelf for the 4-ton ballast that was strung along the electrical cord like a python that had just swallowed a house.
I did some research on flourescent tube lamps a year or two ago and learned that there are several different mechanisms for their death, and the mechanisms depend on different things. The lamp makers try to make the lamp be somewhat near death from all the mechanisms at about the same time, assuming typical usage patterns. The ones I installed in my office assume a usage pattern of staying on for a half hour at a time as part of this. They may be on all day, but often enough I turn them on for a minute to look for something, so it’s probably a fair assumption.
The new fears about mercury in the environment have pushed lamp makers to shift the design to need less of it, changing many of the compromises.
I think all the rules about how long to leave lamps on between uses are trying to minimize overall cost, including energy, replacement part cost, labor, and maybe other things like switch lifetimes too.