MEANS NOTHING!!!!!!...
sorry peeps... but yeah... what we want as reefer's is PAR... it means pennetration of light or something... anyways... that's why 108W of t-5's will be better for a tank lit by two 150W metal halides... why you ask... because the light emited by the t5's is more than two single bulbs on a linear scale...
my friend's tank is a 80g tall lit by two 400W 10,000k SE bulbs and two 96W acnict pc's... i know that's overkill but he SPS don't grow as fast as my friend's 100g tank getting lit by 216W of T5's... srsly...
anyways... WPG means nothing... you guys need to focus your thinking about the amount of light that hits the sand... get a LUX meter...
that is all...