I use a 11W PL-S in my rom as the main light source I once compared it's light output to incandescent by eye, it beat a 75W incandescent

Now i checked the range of PLs available in a nearby store, and there are :

PL-S of 7/9/11W that all run on 1 ballast

PL-D's of 10/13W that run on 1 ballast, 18W and 26W

PL-S of 13W, approx. the same size as the PL-S 9W, which is 13W (and does not run on the same ballast as the others) yet its light output is less than the 11W and close to that of the 9W PL-S or 10W PL-D. I remember that i did not see this lamp few years earlier so it appears to be quite new addition

So : There are PL-S 9W which is same form factor and not far behind light output, PL-S 11W which has way higher light output, and PL-D 13W which is shorter length and higher light output, and the 13W PL-S seems to be a new addition, what was it introduced for, if the other PL's appear to be more efficient ?

Hope this helps :

Quote from Medved :

"The PL-S 13W (~850lm) is US lumen and size equivalent of European PL-S 11W (900lm), so Kev, you will not find one here.
The European PL-S 11W IS ballast compatible (=use same 0.17A ballast) with the 7..9W range (and all 4..13W T5, PL-C 13W and below).

The reason for the US using higher wattage and different ballast is, then the European 11W has too high arc voltage to operate on 120V mains only with simple series choke. So the lamp had to be redesigned for lower arc voltage, what ask for higher current ballast (0.29A, so the US 13W is not anymore compatible with 0.17A 5..9W). The US version ask for higher wattage, because the lower total arc voltage mean the fixed ~10V non radiating cathode fall is of larger proportion of the total arc voltage, so reduce the efficacy. To compensate for this (so get the lumens equivalent as desired), the power input had to be increased to 13W compare to 11W European original. "

Seen in this post.

That this lamp is sold here means that fixtures that use this lamp on 240V exist

Wouldnt it be better to use the 11W PL-S or 13W PL-D in the 240V variant of those fixtures (with the correct ballast), and the 13W PL-S in the 120V variant only ?

Ash: The PL-S 13W is an american 120V lamp, and it isn't available outside North America. Its electrical characteristics are significantly differenet than the rest of the PL-S range (I wouldn't considered it as a part of the PL-S range of CFLs because of this), and requires a dedicate ballast for PL-S 13W lamps.

That this lamp is sold here means that fixtures that use this lamp on 240V exist

Wouldnt it be better to use the 11W PL-S or 13W PL-D in the 240V variant of those fixtures (with the correct ballast), and the 13W PL-S in the 120V variant only ?

I think the PL-S11W is the best. The PL-D's are less efficient, mainly because the quad tube concept let three tubes obstruct the light...

Maybe the single F18T8 ballast powering two PL-S13W in series (it would be about the correct current) would yield lower losses then two PL-S11W ballasts, but I don't know, how much it would compensate the lower efficacy of 13W compare to 11W tubes.