At full load inverter generators are really no more efficient in general. Where inverter generators shine is as the load is reduced from 100% down to some load significantly lower.
Look at Onan diesel comparison:
3200 Watt RV Diesel (standard non-Inverter)
.12 gallons / kWh at 100% or 3200 watts
.19 gallons / kWh at 50% or 1600 watts
6000 Watt RV Inverter Diesel
.12 gallons / kWh at 100% or 6000 watts
.13 gallons / kWh at 50% or 3000 watts
Notice the efficiency of the standard diesel generator drops significantly at 50% power, whereas inverter generator efficiency remains almost as high.
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