I have a 30 gallon hot water tank in a cupboard. The switch to switch it on and off is inside the cupboard. I only use the water in the tank for washing up. But herein is the difficulty, I tend to forget that it is switched on and keeping a 30 gallon tank constantly hot unnecessarily is expensive. I do not own the property so I cannot interfere with the electrics. What I am trying to find is some kind of sensor device that goes around the cable leading from the switch to the tank. If the cable live ( this only happens if it is switched on, of course) then the sensor device could operate a led outside of the cupboard and I could then see, at a glance, if the tank is switched on, or off on the inside. There must be such devices, but I cannot identify what they are called,or who sells them. Your comments appreciated