@LlamaMountainStudioArts
Does that heater have settings or does it always run at 390W?
This is a hard solar task, running a heater in December.

Anyway, according to
https://dotwatts.com/solar-battery-charge-time-calculator/

800Ah and (I'm guessing) 6 hours of good sunlight in December means 16 panels (!)

Note it's possible to add panels any time. Better to get the batteries at one time.
/nosanitize