This statement doesn't mean what everyone thinks it means. Here is why.
Take a kitchen with 9 lights. Install one CFL into one socket and 8 incandescents into other eight. Use all lights at the same time. Which is more likely to happen - one CFL burning up, or any one out of 8 incandescents? This is a trick question based on statistics. The answer is the more incandescents you claim (8 or 8,000) the shorter their life time is - opposite to what most people feel.
The reason is simple. If we read the claim again we will notice that they claim exactly "8 incandescents" - and they don't say that they are used one at a time, and they don't say that if one burns out then it's still OK.
This means that if any one of this large number of incandescents burns out then the test is over, and CFL wins. If you install a million incandescents then, according to the bathtub curve, the probability of early failure grows fast, whereas it is a constant for the CFL. So if you do enough of those tests you will find that a single randomly chosen CFL on average outperforms a million of incandescents. I'm sure you won't need to test for long because there ought to be at least one bad (DOA) incandescent bulb out of a million.
Mathematically, I think, it can be written down this way. Let Cw be the probability of the CFL not failing, and Iw be the probability of an incandescent not failing, and N is the number of incandescents. What is the CFL reliability that matches a bank of N incandescents?
Cw = Iw ^ N
So if the Iw is 0.999 (the chance of failure is 0.1% over the test period) then Cw would be 0.992, and the chance of failure of a CFL would then be 0.8% - about 8 times as high as the incandescent.
Very interesting- thanks Greysard.
Yeah, there was some small print on the back of the box briefly explaining how the claim was calculated, but it’s nice to have more details. In an age in which the Gov. is using various accounting tricks all over the place to say that we’re still solvent and “saving” money on different programs, I guess things like this shouldn’t be that surprising...
I’m just flustered that the CFL bulb already has to be replaced.
I think you just made that up!