Off-grid solar system
Please provide as much detail as possible.
I assume this is for a course etc - and not a real world system? 

What is the location (it matters)?.
How many days without sun must it operate for?
Is there any backup power source (alternator etc?).
What sort of batteries?
How far from panels to load. 

IF you are using an MPPT controller then 12V or 48V system is not too important load wise - a 48V in 12V out MPPT controller is not unusual. However, the system you need is a substantial one. As shown below you need 10 to 15 kW of panels!. At 12V that's 850 to 1625 Amps [!!!]. You do not want a 12V system.  Even at 48V that's 215 - 400 Amps. An even higher voltage system may be in order. 

900W x 24 H = 21.6 kWh/day.
In many locations winter sun is 2-3 hours/day of equivalent full noonday sun.
So your PV system needs to be 8 to 12 x as many Watts as your load.

eg 900W x 24 hrs = 21.6 kWh.
To obtain 21.6 kWh in 3 hours a PV system must store more than 21.6/3  = 7.2 kWh per hour or >= 7.2 kW panels.
Allowing for battery charge efficiency, conversion efficiency, MPPT efficiency ... you'd need 10-12 kW of PV panels.
For 2 hours per day full sun equivalent (many locations in winter_) you'd need 50% more again - say 15 kW of PV panels !!!!. 

That's for ONE day of 24 hr use. For say 3 sunless days you need 3x that = say 30-45 kW of panels [!!!!]. Battery also gets "interesting".   

MORE DETAILS PLEASE.