How to determine the efficiency value of the inverter we design?
The simple answer is yes to both questions. However you must calculate the power using a sufficient number of instantaneous values of the output voltage and current and integrating the results. That is necessary to avoid errors due to phase difference between voltage and current and harmonic content. The calculated efficiency will only be valid for the load condition used. If the inverter has forced cooling, you must be sure to include the power required as part of the input power.