For very rough calculations
, I use about 80 - 85% as an assumed inverter
efficiencies are typically specified for resistive loads, at mid-point (40 - 90%) output power, and can range from about 80% to about 95%.
Inverters are much less efficient when used at the low end of their maximum power.
Modified sign wave Inverters are also much less efficient when operating motors, than are True sign wave units.
Both of these operating conditions will further reduce the total efficiency.