I did a spread sheet for power usage
Power Consumption / Light Cost that I think is prety cool. Check it out.
You can/should use floresents for you 18 on veg period this will help with power during those long days of light.
As for the 400 being overkill no way, you will be quite pleased with its results.
If your gona let them get tall you may also want to do a string lattus at about the 3 foot level in the cubord to keep the stems spread out and geting even light.