There are many reasons why the losses might be higher or lower than one is expecting. Without a model or knowledge of your application, it is very difficult to say why. It would be more helpful if you could post your complete model or a simplified example that shows the relevant features and the results you were expecting. If you don't feel comfortable posting to the forum, you can always reach out to email@example.com for assistance if you have a PLECS license under maintenance.
With that said, my initial impressions from the limited data available in your screenshots and description:
- The formula you entered above has a typo. It should read (Pin-Ploss)/Pin*100 or (1-Ploss/Pin)*100. I suspect it's OK in your model as your efficiency reading is reasonable if Ploss << Pin and Pin >> 1.
- Since you are working with a 1PH AC inverter then the instantaneous power losses might be small at certain operating points (e.g. near 0 current) but high at other points. Ploss should be averaged over a line cycle, as should Pin.
- Are there thermal descriptions associated with your components? If not, then the thermal losses will be calculated based on the On-resistance parameter of your MOSFET with diode. There also is the possibility that the thermal description is inaccurate in some way or you are not modeling another dominant loss source in your circuit.