they say 6-8 watts per gallon, but different lights have different lumen output so the real question is how many lumens per gallon and at what light spectrum?
say you use 48" VHOs (110Watts) rated at 5500 lumens then you decide on VHO
internally reflected at 40% more output say 7700 lumens and you have a 110 gallon tank then at 6-8 watts/ gal
you would need 6-8 lights giving you from 33,000 to 61600 lumens and burning 660 to 880 watts of power.
T5HO burn at 54 watts and 5000 lumens and with their special 9 bend reflector that is advertised at 300% increase in output gives you 15,000 lumens per bulb at 54 watts instead of 7700 lumens at 110 watts per bulb.
61600/15000= about 4 T5HO bulbs with a total of 216 watts of power instead of 8 bulbs at 880 watts of power. Also T5HOs reported to last 18-24 months and VHO
P.S. metal haildes follow a different set of rules based on the depth of your tank. mainly 1 every 24x24" square and the deeper the tank the more power you need.
have fun shopping until your head explodes and then just punt, guess, experiment and spend spend spend.