Thanks for the reply's, I'm thinking of leaving the existing 150 amp main panel alone and then upgrade the existing meter to 300 amp and then feed a separate 125 amp main panel just for the heater.
I doubt he will want it done after he sees the cost, I just don't understand why they would make these things so big unless they think every house just has a spare 125 amps to play with.
Here is a link to a similar unit that is 22kw
http://www.tankless-water-heater.com/products.html
