PV solar panel inverter
Assuming that you have enough energy available to supply the load at the chosen voltage then you do it the same way as for any regulated power supply. 

The most common method is to sample the output voltage, divide it down to a more convenient lower voltage and use a comparator or functional equivalent to compare it to a reference. 

In the case of AC you COULD create an AC reference but this is harder than is needed. Instead, rectify the divided output signal and compare it to a reference DC signal and then provide a higher step up voltage if output is too low and the provide a lower step up voltage if output is too high.

Your question may not be exactly what is intended. If the above does not answer it please be more specific.