I'm not very good at understanding lighting, even with the info provided. It just doesn't click for me.
Can someone tell me how 24w of T5 would compare to 36w of PC?
A 24 watt T5HO is about 24 inches long, so it can cover a 24 inch tank very well. A 36 watt PC is about 17 inches long, so it leaves the ends of a 24 inch tank darker than the middle.
The reflector is a very critical part of any light fixture. T5HO fixtures almost always have excellent, highly polished aluminum reflectors, so most of the light the bulb produces actually reaches the aquarium. PC fixtures, other than AH Supply, tend to have much poorer reflectors, so less of their light gets to the aquarium. On a 16 inch high tank, the typical T5HO, single bulb fixture should give you about 100 micromols of PAR - high light - while an AH Supply light should give you about 70 micromols of light - medium light - and many PC fixtures would give you about 30-40 micromols of light - low light.