You can run a 1500w resistive heater for 50 hours on a 75kwh battery. I run a 1500w heater in my 30ft RV and it holds comfortable temps down to freezing, in a

You can run a 1500w resistive heater for 50 hours on a 75kwh battery. I run a 1500w heater in my 30ft RV and it holds comfortable temps down to freezing, in a small vehicle it should have no problem even down to very low temps, 750 watt might be plenty to keep the interior at a safe temp.