watts/gal is really not a terribly valuable number anymore. There are a lot of people with experience keeping just about everything and they would be happy to give you an idea about your plans. I assume you are thinking about an anemone since you posted to this forum.
You won't need MH for anything that will fit in your 10g tank. No anemone should be kept in a 5.5gal. IMO
If you want the MH for SPS and just want to add a BTA (the only thing I'd recommend for a 10g) it will be quite happy with MH.
Oh, I forgot, some folks like to keep condys. I've never kept them but they don't really need much light IIRC.