Why do we need a over head tank. Instead we can install a new kind of Tap which can be a Electromechanical component. What I mean hear is, the tap should act like a Switch to the motor pump, and also functions as a valve to let the water through it.
In this case the motor will switch on when we open the tap and the water can flow directly from the motor out-put than from the over head tank.
This is the reason I think this will be of help. In case u want to supply water for the second floor in a 10 store building, we are only using enough energy to pump the water from water surface to second floor. In our current method, we are using energy to pump the water for 10 floors and using the stored potential energy to get the water from 10th floor to second floor.
This can sure be one of the solution to save power.