Think about it for a second. Your inverter is rated for 1800 watts which figures out to about 15 amps. Your generator is probably 4000 watts (or maybe 5500 watts), which calculates out to 33 amps at 120VAC (about 45 amps for a 5.5kw genset). Many users would not pay attention to the limitation and as a result, they would be constantly trippjng breakers until the inverter went belly up! So the manufacturer has tried to help avoid such things by limiting what can be connected (within reason). Also the heavier load would discharge the batteries much quicker.
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