I gotta jump in here ...
It's about lumens, not watts.
Yield per plant has so many variables, I wouldn't dare give an estimate with what little info has been provided here.
As a rule, the more lumens you have, the better your crop will be. But, you
can have too much light which can damage your plants, cause ladies to turn hermaphrodite on you, and all sorts of other problems.
I've grown a plant using a 100watt standard light bulb, veg through flower, and managed to produce a bit of weed that did the trick. But that was 100 years ago. Today, we have so many options and products from which to choose, it doesn't make sense to waste time, money, and energy on anything less than "decent lighting".
Take a look at this link for some great newbie lighting advice:
http://www.growweedeasy.com/grow-lights It's written by "Nebula Haze" (username) who always has good advice, imo. He doesn't just tell you what to do; he explains the reasons for doing it, etc.
Go forth and learn! Then enjoy the fruits of your success!