Scoured the forums but couldn't find a clear answer - how efficient is the dimming feature on a ballast? For example: A decent 600w HPS puts out 90,000 lumens, 150lm/watt. Dimmed to 400w, (400*150) = 60,000 lumens? My assumption is that unfortunately there will be a bit of lost efficiency, maybe dropping it down to 120lm/watt or worse. Does the dimming feature also reduce the HPS bulb lifespan? Would it be better to buy a 400 watt bulb and then dim it to 400w?
It depends entirely on the model ballast you are talking about. Some are very efficient, others are not as efficient. An inefficient ballast or badly designed dimmable ballast may shorten the bulb life. Good questions!