...by that logic if my 100W bulb is running on 10V, with my same 20Ah I'll get 200Wh, so I'll get a bulb of the same wattage, with the same battery, running twi

...by that logic if my 100W bulb is running on 10V, with my same 20Ah I'll get 200Wh, so I'll get a bulb of the same wattage, with the same battery, running twice as long? Magical double energy! ...this is why we should measure the total energy in Wh, not Ah.