how big is the closet?

I would have thought for any closet, a 400w would be plenty.

I guess if you have 3000 lumens or more per square foot of grow space, you're fine.

and with a 400w HPS you want to be able to change the air in your grow room 3-5 times a minute. To calculate your room size, multiply Width by Length by Height, this will give you the CFM rating required for one room change per minute. Times that by 3 at a minimum and you have the minimum required CFM rating you need.