Is the cable size for a PV system connecting back to the consumer/distribution board calculated the same way as for any other circuit. Check the load/current that the circuit will be drawing and make sure that the cable's current carying capacity is the same or greater. Then check the voltdrop is within the permited levels.
In PV system's case, see how much AC and DC current the PV system will be supplying and make sure that the cable current carrying capacity matches it or exceeds it.Check the volt drop in the same way?
Also, if its a grid tied system. Does the current from PV system flow through the incoming DNO's fused cutout ? How do they deal with sites that have large PV and the site's generation current is bigger than the fused cutout size?