My HPHW water heater costs me between $5 - $7 a month to run. The HPHW I just installed will be even cheaper since it has a UEF of
3.45. I'm sorry but there is no way an on demand electric unit can make hot water that cheap. I doubt your can even make it that cheap with natural gas.
Simple electrical match ( V over I,R):
80 amps x 240 volts =19,200 watts
19,200 / 1000 watts ( 1kwh ) = 19.2kW
19.2kW * .14 cents ( my cost 1kWh of electric ) = $2.688 per hour to run
At $7 a month I'm using around 50kWh of electric all month to make my hot water.