I am just curious as to what use the capacitor has in modern HPS fixtures?  I am only asking because in 100W or less, there is no capacitor, while in 150W+, there is.  Why do 150+Watt fixtures need this extra piece of equipment, but 100-Watt do not?  Another reason I ask is, this part seems to fail more often than Ballasts fail, at least in cobraheads.

Here is a pic of an older oval style GE Capacitor that I recently replaced.

It would not be necessary with any wattage as long as the system stays low power factor

What i dont get is, to correct the power factor a capacitor can be added in parallel with the line input (same as its done over here), so even when it degrade, it won't affect the working of the lamp. Why in US ballasts they place it in series with the lamp instead ?