Check first what type of water heater you are using. Instant water heater or tank type.

Just take a photo of your shower area. It will show whether you have a instant heater or a tank type. Funny that you cannot differentiate between the two.

Perhaps you can use the tank type until it finally konk out and change to instant water heater which to me is more practical, uses electricity only when water goes through the magnetic switch and hot water flows almost instantly. I guess the tank type is more when you need hot water for whole house such as kitchen also.