8.64 joules = 2.4 mW hours. Dividing by 5v gives 0.48 mA*hrs, so you can run a 5v load for 1 hr if it takes 0.48 mA.
Take the easy route when it's presented like that! 2.4 mW hours is 0.1 mW days, dividing by 1 day (i.e. doing nothing / applying the "per day" part of the original units) and 5v gives you 0.02 mA.
You could also just go back to the sentence just before "8.64 joules of energy per day" and see that it produces 100 microwatts (or 0.1mW).
Ampere-hour is a unit of electric charge, not energy
5V amp-hours is watt-hours, which is joules, which is energy.
you are right, somehow I skipped the "5V" part. My bad.