Hello, I am a Canadian Mechanical Engineer with close to 20yrs analysis experience. I will mention up-front that your concept might be impractical... To save money on water heating, there are commercially available "instant hot" water heaters that are extremely energy efficient and function only "as needed". That said, nonetheless, there could be a market for an inexpensive retrofit to common hot water tank systems. I believe your project needs a bit of research because it is easy to say "turn the heater off when it is not needed", but then when off, the water in the tank will gradually cool. There will be an ideal schedule where the energy needed to re-heat the water will be less than the energy needed to maintain it hot. Another complication, is how much inconvenience a consumer will tolerate? What if on rare occassion, he/she returns home late from a party or night out and needs a shower to wash off the night's grim... no hot water... Again, the application might be impractical for residential use... but maybe for office buildings and the like, where schedules are much more predictable. At any rate, I am more than willing to lend my consulting expertise to you; I wish you great success with this "green" initiative!