How much hot water is in my tank?

Did you know, 20% of your home energy bills comes from heating your hot water (SEAI)? Yet until now, there has not been a visual way of knowing the amount of usable hot water in your hot water cylinder. So, there is a tendency to set the schedule on a hot water controller for late afternoon/early evening and blindly heat the water for a few hours with the hope you have enough hot water for the household. In general, there is little or no consideration given to heating too much water and the subsequent wastage of energy.