I have been looking for an answer to the total generation
I would think that each engine has a 120KVA generator on it, a minimum of 90KVA would be minimal standard. Tried a google search and failed. None the less 90KVA=360KVA total would be what a 744 would be capable of. What they allow for personal electronics, I do not know. Most likely 480KVA is available, 20KVA/2000VA would allow for 400 pax to draw 5 amps each, 5 amps is the standard for a pax plug/lav power. There may be a shedding programme but none the less, it sounds like all should be provided sufficient time to charge an electrical device.