Maximum load power=120*8.5=~1kW
Assuming 75% inverter conversion efficiency, input power =1kW/0.75=1.3kW.
Assuming minimum battery voltage of 10V, battery current=1.3kW/10V=130Amps.
Total battery capacity=5*3 ampere hours=15ah.
So running time=15ah/130A=0.1 hours (an approximate figure based on assumptions made)
At the discharge current needed, the battery voltage drop will be high, the self heating of the batteries due to the internal resistance would likely damage/destroy them.
Charging multiple batteries in parallel has its problems as well, you would need to do some reading on that topic.
High capacity 12V deep cycle batteries typically have ratings of hundreds of ampere hours, compared to the 15 you have available, they are however, heavy, large and expensive, (the price you pay for ampere hour capacity) but one battery would give many hours of use,
cheers,
Richard.
