For plants PAR is the best, lumens comes next, and Watts is the poorest.
PAR requires either paying for an expensive meter or borrowing one from someone.
a Lumen meter is cheaper but much less accurate for the planted tank. Some might argue it's not really usable.
Watts buy itself is almost worthless, however coupled with the temperature and type of bulb experienced hobbyist can make general suggestions that will get plants growing.
There is no exact calculations as to the amount of light that plants need in tank. Each species has it's own requirements and ranges of tolerance. For most of us it is a slow process of trial and error to learn how to grow them. Some people get lucky and make a beautiful well balanced tank on the first try. It is very proper to hate these people.
I currently have 1 beautifully balanced tank, 1 algae farm, and 1 absolute disaster. The trick is to spend most of the time watching the beautiful balanced tank.
 
	 
	