For
very rough calculations, I use about 80 - 85% as an assumed
inverter efficiency.
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.