RBM remove this wrote:
There is a difference between models for heating water and domestic water,
there are also a number of sizes and wattages. You need to determine what
you're going to use it for, and how much water you need it to produce, then
you need an electric service of adequate size. Some domestic units run at
28KW, or 116 amps @240 volt, which would require your house service to be a
minimum of 200 amps



Actually you would need TWO mains one just for heating water!

116 amps or water leaves little to run the entire remainder of home
which is today generally 200 amps. you dont want to be at 200 amps all
the time.

you will need a pair of 200 amp main services one just for heating water