Hi,
You mix a lot of things and miss to give important informations.
A pedantic view on your given informations:
Wire size depends on current (A) not on power (W). Wire size is never designed for Watts.
(You may calculate current from power, but for this you need to know Volts and in case of AC you additionally need to know power factor: I = P / (V * pf )
kWp means kilowatts peak (which is power)
kWh means kilowatthours (which is energy). An inverter is not designed for kWh at all.
Circular area is A = r * r * Pi, where r is the radius (half the diameter) and Pi is about 3.1416
The inverter has an input and an output. But carry different currents, thus need different wire sizes. You need to meantion which one you are talking about.
And in case of grid side cabling you need to tell whether it´s a 1 phase or 3 phase system.
Klaus