Ensuring stable water availability : Need advice on choice of pump

We live in an apartment which is a 6 storey building with 26 families in residence. We have a 3 HP motor and sump below which is about 5 meters deep. There is an overhead tank on top of 6th floor (Height could be 90 Feet from Ground). The pipe dia from sump to the motor / pump (inflow) is about 2 inch and pipe dia from pump to the overhead tank is 1 inch. Currently we have to run this motor almost 18 hours a day to ensure water availability.

In case of power blackout water is not available as the pumping capacity is more or less in sync with the usage. Replacing the pipe being cumbersome, what could be the optimal motor capacity which would effectively use the available pipe and ensure better water availability. Any other parameters if any that I should keep in mind regarding the purchase of a new motor?