if an ipad has a 40Wh battery and can run for 10 hours then it consumes 4Wh per hour so runs at 4W or 4J/s. 100kg dropping 1m releases 1kJ so will supply an ip

if an ipad has a 40Wh battery and can run for 10 hours then it consumes 4Wh per hour so runs at 4W or 4J/s. 100kg dropping 1m releases 1kJ so will supply an ipad for 1000/4 = 250s = 4min. so a 100kg weight needs to drop 15m to power an ipad for an hour. i don't completely understand the parent comment - what are "these"? but you do not need multiple weights and shafts to run an ipad for an hour. a single 100kg weight and a 15m shaft is sufficient.