From capacitor charged 12V, 0.3A drawn, how many farads needed to last 24 hr and still have 9V?

Q(CHARGE)=C(CAPACITANCE)* V(VOLTS)...AND THE TIME CONSTANT IS R*C MAY BE THIS WILL HELP YOU....I WANT TO KNOW IF THE CAPACITOR GETS CHARGED WITH DC OR NOT..PLZ ANS ME AT nirbhay.pandya@gmail.com