Water heaters come in many sizes. Look for the wattage of the heater and use the following formula to find the current. I = W/E = Watts/Volts. As you can see a voltage and wattage of the tank is needed to find the current.
At current gas prices they cost less to use and a gas heater gives you a much greater supply of hot water on demand.
A tankless water heater is much more efficient as it "flash" heats the water.
Very time consuming job. It takes what it takes.
The installation of a solar water heater will cost between $250-500.
None.
You constantly need hot water
Not advisable. Both units consume huge amounts of Amps (amperes = current) and the normal 20 or 30 amp breaker may not sustain prolonged operation of both. Where I live, we have two electric meters ... one just for the water heater, the other for the house and all it's electrical needs (except for the water heater).
4.12A. V=IR
It depends on what is broken on your water heater, as to how much it would cost to fix it. It might actually need to be replaced and be more cost effective to replace it.
The formula you are looking for is I = W/E.
Not much, it gets warm as everything else within the heater's range.
Aside from heating water, pretty much nothing.