I just connect my LEAF with the EVSE that comes with the car to a 20A regular outlet at a campground to charge my car overnight. It makes a significant difference.
I believe bigtlb's estimate is fairly accurate based on my test:
The easiest way to remember is: 1% battery consumption for every 100m elevation gain for a Leaf with a 40 KWH battery (e.g. my 2019 Nissan Leaf S).
