I need to ballast a 100 watt HPS lamp and I cannot decide between a NPF ballast w/no capacitor or a HPF ballast w/capacitor, both of which are 120V only. I would like to go with the NPF as it is cheaper but would I be better off with the HPF and why? Is there a difference in lamp performance/life between the two? It is going inside a compact floodlight and there is plenty of space, just don't know which one I should go with.

Thanks for the input.

With a standard reactor ballast, the capacitor's only roll is for power factor correction and has no effect on lamp performance. If you were to go to the store and look at an NPF ballast kit and an HPF ballast kit, you would find that the core/coil are exactly the same (probably right down to the part number) and the only difference is one kit includes a cap and the other does not.  That said, you could buy the HPF kit and leave the cap out when you install the ballast, or buy the NPF kit and add a cap later if you choose.

The difference would be only on thr current load of mains wiring: Without the cap it would use about 2.5..3A, with cap about 1A. But using the capacitor has a drawback: You would have huge, (but very short in time - charging the capacitor to the actual mains voltage at the moment of switch ON) inrush current, when you switch ON the light.
So the cap i would recommend only if you have long cable feeding the light, where the larger uncompensated current draw would create unacceptable voltage drop on the long cable resistance. With such long cable the inrush current would not be a problem, because the same cable resistance would limit it. But if you are using it with short wiring, there would be no resistance in the feeding path, so the higher current should not be an issue and the capacitor inrush current would wear switch contacts. Furthermore if some higher frequency disturbance would be superimposed on the mains voltage, it would cause very high capacitor current, possibly even tripping the breaker or fuse.

Wow! That is great information and that is  exactly what I needed to know, so NPF it is!

I really appreciate the help. This site should have a ballast expert section so one could go there and get expert advice, like from both of you. Maybe also a lamp expert section too. These kind of things don't seem to fit in the general discussion and with so many members on here that have expert knowledge it would be a real time saver if someone needed this info and knew exactly which thread to look it up in.

Yeah, the only reason to go with HPF is if you have a LOT of wattage to deal with. Like in commercial or industrial establishments, only half the copper has to be used for wiring which likely offsets the higher cost of a capacitor.

Also probably not a good idea to run a NPF 1000W 120V ballast (do they even exist?) on a 15A circuit for example.

I think running 1kW HX ballast on 120V/15A breaker would not be a good idea at all, as when the arc extinguish (e.g. due to power micro-cut), the capacitor would draw about 20A before the lamp restrike. Due to extinguished arc the inductive current component is missing, so the full capacitive, normally compensated out by the inductive, would flow trough the mains wiring, so tripping the breaker soon.

Cool, just ordered a Venture NPF ballast, thanks again for the help.