Power in = V * I * PF
Power out (of ballast) = Lamp brightness
At first lets disregard ballast losses completely. If so, V * I * PF = Lamp brightness
If a ballast have lower I, same V and PF - lamp will be dimmer
If a ballast have lower I, same V, but higher PF - that would depend on how V * I * PF actually compare. Not all HPF ballasts are necessarily equal, one might be PF 0.9 and one be PF 0.99
If a ballast have lower I, higher V (say, one rated 110V and one 125V) - that means that it is meant to draw lower current at its rated higher voltage. What will happen if you power it with not as high voltage ?
- Magnetic ballasts will tend to lower I with lower V, so underdrive the lamp three fold : Once due to the lower rated I to begin with, Then due to the even lower than rated I resulting from the lower supply voltage, and then due to the lower voltage too
- Electronic ballasts vary. Some will behave similarly to the magnetic, some others will actually increase the current at lower supply voltage, so might drive the lamp just the same
If then we look at ballast losses too, then the difference in ratings might also be attributed to that. For example, one ballast have higher V * I * PF, so higher input power, but happens to be more lossy, so the output power could be the same or less than the other ballast
PL-C 18W ballast is good only for PL-C
PL-L 18W ballast is good for :
- Magnetic is good for 18W/20W T8/T12. With 120V ballast thats about it. With 230V ballast, it is also good for several other lamps
- Electronic - usually 18W PL-L ballasts are also rated for 24W PL-L and maybe other sizes, and then they can drive also T5HO of the same wattages as the PL-L
15W lamp is of lower arc voltage than 18W, so not necessarily suitable for all ballasts. If its magnetic, see how much it heats
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