I have been thinking about my lighting - in particular lack there of....

Currently, all I have are SB900 (Nikon) and two Metz 45CT-3. I also have a set of small umbrellas and stands. What I like taking is environmental portraits.

I am aware, Watts/Seconds and GN are totally different measurements and they don't really compare....

I am currently looking at Alien Bees units. B400 is 160Ws, and B800 is 320Ws. How do these compare to what I have in terms of light output? Let's say I have SB900 pointed to my small umbrella and B400 pointed to the same umbrella. How's the output in comparison?

I know we have portrait experts in here.... Are there any rule of thumb for environment portrait such as "don't bother with such a small unit!" or "you are way overkill...."

Or - are there recommendable units rather than AlienBees? I know these are considered a lower end Volkswagen kind of units.

I'd appreciate any input.