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Ok the problem with the tankless is the lines are going to get hot in response to those 80 amps.
The ideal situation is one in which the tank heats to its max temperature and is used at approximately the same time as the heat finally convects through the insulating materials.
blah blah blah. . .yeah
You want your water to heat up the same way you take off from the red light. Mash the gas you get there faster, in shorter time and whatnot but you still burn more fuel than if you do it slowly. same goes here. Do it slowly and you save energy. Too slowly and you never get there(drag overcomes acceleration forces) and the heat dissipates as fast as you "make" it.
A new energy star efficient tanked water heater can do a full tank(30 gallons) in about 30 minutes to max temperature. You want to set a timer so that it stops heating the instant you go to get in the shower(or if there are multiple people in sequential showerings then halfway through the whole process) because the elements will still be pretty hot for several minutes after you switch the juice off. This allows the water not to be heated anymore than what you need no faster than you need it. Also if you use all the water and leave the bathroom door open in the winter its going to dump BTU laden warm air into your dwelling providing BTUs to the rest of the house you already paid for but will get wasted if you turn on a bathroom vent.
Also using a cystern inside your house to acquire local heat is only effective in the summer. If you are paying for heat you are paying for it one way or another(given if you use a heat pump it is more efficient to heat with a heat pump than electrical(coefficient of performance is usually around 2-5 depending(for every 2 KW of juice you put in you get 4-10 KW of heat)), but if its summer you can use this technique as a "passive" cooling device that preheats your water to avoid heating losses.
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