I assume you mean the inverter drew 8-9 amps on the AC side of life. Thus making it a MEDIUM (Dang near a BIG) load.
Mine can hit 11 (Big load) if the batteries are low.
In fact if you read any of my posts on the limits of 30 amps I mention this.
Output current is aroudn 800-900 percent of input current.
So if it's drawing 10 amps from the mains, output can be around 80-90 amps or less depending on the type of charger.. LESS.. Mine hits 11 amps at 80 amps out.
The math is fairly simple.. Watts = Volts Times Amps (DC) or Volts Times AMps times Power factor (AC) for this we will bundle the power factor with the convwersion loss so Watts = Volts times amps
Watts in, Times conversion factor (Effiency) = Watts out
So if you are putting out 100 amps at 12 volts. You need at the very minimum 10 amps input at 120 volts.
That is simplified, Output is likely 14 volts and change.