Its not MH or T5 and even then consider the reflector...
I've measured PAR of Cheap Chinese Imports at slightly over 130 watts with a killowatt and apogee Par Meter around 250 PAR in free air and then I measured an american fixture at 150 watts with over 700 PAR at 24"...A 400 watt MH Raduim gave us a 423 PAR at 24" using a reeflux reflector. The MH did offer more spread but for a standard 24-30" tank this was moot.
Led Type, Specific Bin, Optics, Drive Current and Thermal Management all effect the amount of light your corals receive.
My Apologies on being terse but asking how many watts per gallon is missing the vast difference in LEDs. Look at LEDs like memory chips a few years back then compound this with optimization of drive current, optics and thermal management.
There are a lot of 3 watt Bridgelux fixtures sold but Bridgelux no longer sells its dated 3 watt LEDs???
If you wanna spend some money on some really good lights get radions . There pricey but well worth your buck . You would need 2 of them depending on the size and fun of your tank . But I sure you will need too.