Capacitance of a circuit
To determine the appropriate rating for an electrolytic cap, you have to analyze the circuit to estimate the maximum voltage across the capacitor on all modes of operation and add a suitable safety factor. 

In "many" cases the maximum supply voltage will be the maximum the part sees, however that is definitely not always true. Counter-examples include boost converters, charge pumps, and clamps on inductive loads. 

As well as voltage rating, you should also consider tolerance, temperature rating/lifetime specification, ESR (Equivalent Series Resistance) at frequencies of interest and particularly at low temperatures, and maximum ripple current, which interact somewhat, in order to come up with acceptable part numbers for a design.