Sorry, should have laid my math out a little better.
Weight of water (8.30#) times the desired temp rise. In my case, I'm figuring 60*. My hot tap is about 125*, so 40* is plenty for strike water. So, 40* times my weight of the water, in this case 33.2 = 1328kw will heat that amount of water in one hour.
I do have the option of putting in two 1650kw elements. That would speed it up, and give me the ability to do larger volumes.
Also, with a submersed element not give me 100% efficiency? Why would you figure 80%?