Sounds like a lot of amps but only when there is water flow through them. You need at least a 3GPM for a small house of two people. Tanks heat all the time. Insulate the crap out of it helps.
Yeh, looks like that would be the 18. Draws 75amps! And yeh I get that it isn't a lot of KWs on the bill, but it still needs big wire and upsize the breaker.