A lower output adapter should still charge the battery, but much more slowly.  And if your plan is to charge the system while using it, you will need a full 65W input to do so. Inverters invariably cause losses, so be sure your inverter can deliver at least twice the output (i.e., 130W to an inverter that runs a 65W power adapter).  
Hello there, I will be working off-grid for a few days and want to charge my laptop using either a powerpack or a powerbank. Is it more efficient in terms of power usage to use the USB-C port for this, rather than using the Dell mains charger with a powerpack with inverter, or does it make no difference? Also, do I need to find a powerbank/powerpack with 65W output, or will the laptop still charge with lower wattages? Do powerpacks/cables need to be Dell-specific? Many thanks for your time.
Charging Lat3420 using USB-C port and powerbank/powerpack