I took a 69-minute ride last week, and ridewithgps tells me how long I spent at each speed. I made a spreadsheet with that data plus the power output of the Shimano hub as specified in the graphic you posted.

The spreadsheet lists some assumptions, some of which we know are wrong, but they are place for starting.

You fill in the cells with the yellow background. I filled in that I need three gadgets fully powered on the whole ride. That won't be the case on all rides. On this ride, I theoretically created a deficit of 4.27 watt-hours. On rides when you don't need the headlight or to charge the smartphone, you would create a surplus of power in the battery.

Tweak the numbers and see what you get. Tell me what you think.