I would suggest that for the first couple months you run open class "anything goes" just to build a good following, then start segmenting into something like this:
Class A - 5,001 watts and up
Class B - 1,001 - 5,000 watts
Class C - 1000 watts and under
These basic 3 tiers should make sure there is always atleast 10 submissions in each class. If you make too many subclasses you may end up with some dead months for a class that doesn't get enough entries.
Cheers,
Cappy