Also, watts per gallon does not take into account the dimensions of the tank.
A 162l tank at 900l x 600w x 400h is going to get different light at the bottom than a 900l x 400w x 600h tank.
It also does not take into account the distribution of light.
A bank of fluorescents that spans the whole tank will give different light at the bottom of a tank than a single metal halide of the same or similar wattage over the middle of the tank (yes I know there are things called diffusers, but this is just an example)