It can maintain the cooling temperature of water and 80% of copper is available.
"How much" as a question is quite vague. Some hot water heaters are comprised of copper only. So this now depends on the size of the hot water heater. Some hot water heaters have no copper whatsoever.
If you are talking water heater, they are made of steel. If talking storage tanks, copper lasts much longer than steel. Will not rust.
You can find more information about how much a water heater installation cost by going to a home improvement store such as Lowe's or Home Depot or going to their direct website.
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.
A tankless water heater is much more efficient as it "flash" heats the water.
For information for water heater installation you should visit your local Lowe's or Home Depot department store. You can find help, ideas, suggestions and much more on just about any product from a qualified customer service representative.
The installation of a solar water heater will cost between $250-500.
None.
Since you cannot remove the "stains" with the methods you have tried perhaps the stains are from too much acid or high acid levels. Are there also copper colored stains as well? Do you have a pool heater in the system? If so, then it is copper stains from an acidic pool water. The over acid conditions are stripping the copper from the heater copper tubing. k
You constantly need hot water
Copper stays where you put it much better than saline water does.
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.