tankless water heater
This heater alone required 113 Amps with really heavy hot water usage.  So first you need an electrician to look at your house and your electrical panel/s to see if you have that much power available.  The sub-panel idea is good if you have the available power.  The sub-panel would need a rating of at least 125-Amps.  You need to have newer and quality service equipment. Otherwise this type of a load could really raise havoc.