The total theoretical demand for a water supply system can easily be calculated by adding known maximum demand for all fixtures in the system. Due to the nature of intermittent use this will unfortunate add up to unrealistic demands for the main supply service lines. A realistic demand for a supply system will always be far less than the total theoretical demand.
Below are the Calculations
mean demand = peak demand / peak factor
Overhead tank size = (peak demand - mean demand) x duration
Transfer pump capacity = mean demand , Assuming one pump running & one standby