Design Calculation of Overall Inverter

Take off you overalls and hang them upside down.

If you want a sensible answer give more details

There are inverters for different applications,with wide ranges in power levels. Load calculations, battery sizing, wiring considerations all vary.

The subject is vast. You should not look to the simplest way especially when you are a fresh engineer. Suggest referring to text books for detailed study. Before attempting straightaway to design calculations, first be specific on the application,i.e.design intention.