Be careful of how much you load down inverter when you have engine running. Also alternator may be hampered on output with engine at only idle. -- As example 1200 watt inverter 120 VAC single phase load of 1200 watt draws 10 amp on the 120 V line -- BUT on DC side will draw 100 amp at 12 volt DC - even more as battery voltage droops. So it is all determined by the LOAD and how long you run load.
Dan Bentler
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