The wattage pulled from the wall is your out put. the manufactures stated wattage is just a marketing ploy.
check this:
http://www.dormgrow.com/wattage
It is hard to estimate anybody's yield, because of so many factors involved.
Not only is the wattage taken into the fact, but the distance from canopy, strain, if topping/lst involved, type of medium, nutrients and experience of grower...etc.etc...
The bench mark was about .5 grams per watt, some peeps have done the 1 gram watt... but really the most important is Quality...
IME... leds cause bushier plant, so sometimes I need to raise the lights farther than I normally would just so I can have space between internodes. hope some of this helps