I assumed that performance curve was based on USB output, thus somewhere around 5 volts. Some of my devices that can draw more amperage than a charger will deliver will draw the voltage lower than 5, but I always assume 5 unless I have a meter hooked up.

I think most powerbanks are also rated in amperage at USB voltage, not amperage at the Li Ion battery voltage.

***

I was in a rush to get out the door when I wrote that posting, I should have elaborated further. When I made my comment on 25 hours or 300 miles, that really is not a realistic measure because everybody will start out a trip with their batteries topped up. And if you are in charging mode starting on day two for the rest of the trip, worst case scenario is that your big powerbank will slowly lose power if you are using power faster than you can generate it. But sometime on the trip if you have a chance to plug it into an outlet overnight to top things up again, you are essentially starting over.

My two week tour this past February, I was self sufficient, never needed to plug anything into an outlet.

But that has not always been the case. In the photo I am topping up two powerbanks and four AA batteries at a construction project where the campground owner left a power supply for me to plug into over night.