When you first start the vehicle with a fairly full starting battery, voltage jumps to 14.5ish but very quickly drops and settles at 13.8ish, at least with my truck.
The inverter on the starting battery trick is to see how much load you can put on the inverter while the vehicle's voltage holds at that 13.8ish. Turn off vehicle loads like climate control.
Happens sharply with adding load. "Last straw" effect. Then dash voltage needle goes from straight up, but shaking a bit, to quickly dropping off to the left. Remove some inverter load and the needle goes back to "up" That is your inverter load limit.