Hi Michael,
You have asked a simple question that requires a complex answer. The
simple answer is a lot.
Please see Section 7.9 in the Car and Deep Cycle Battery FAQ on
www.batteryfaq.org on how to size the batteries, inverter and charger
for your application. By "car batteries", I assume you are talking
about the physical size and weight, e.g., Group 24 or 27 because you
should use deep cycle batteries for much longer battery service life.
The most important conversion is the 1650 watts (12.1 amps) at 120 VAC
to 145 to 169 amps at 12 VDC, depending on the efficiency of the
inverter and loss. You might need to increase the battery capacity
due to a lower temperatures, higher current rate due the Peukert
Effect, and Depth-of-Discharge based on battery type.
Kindest regards,
BiLL........