Need information on water pump selection for domestic use for a house in Delhi

Dear Sir,

I am living in north delhi and have municipal water supply pipe in my house.Actually i m confused How much HP motor i will need to fill my tank which is on fourth floor??Earlier i was having a crompton greaves motor of 0.5HP and the tank getsfilled within 30-40 mins.the flow of water was good in tank(0.5inch pipe is used to fill the tank from the motor infact all the connection in my household is through 0.5inches pipe only).Few days back i had shifted my tank one floor up.Now the amount of water through my  pipe is very less.It almost takes 4-5 hrs to fill my 500ltr tank.The plumber says to replace the motor.

I am little bit worried about my electricity bill too.I want to request all the experts to please guide me which company and how many HP pump should i buy?Should i replace the 0.5inches pipe with .75 inches pipe and install a higher HP pump?Or the plumber has done somehing wrong like filled some sand or garbage in pipe and wants me to change the pump??

Thanks in advance.