"Scientists at the energy group's technology centre in Nottingham aim to build a large-scale prototype that would be able to store one megawatt of electricity for four hours - the equivalent to 10m AA batteries and the same size as four articulated lorry containers."
Not that it matters any, but I think they got it wrong. If you "store one megawatt of electricity" for four hours, what they mean is that it will store four megawatt-hours of energy. The correct way to say this is that the battery will provide one megawatt of power for four hours.
But that is just semantics. What is incorrect is the comparison with AA batteries. A typical alkaline AA battery will provide 3000 ma-hours. That is 3 amperes for one hour or 1 ampere for 3 hours (or any combination of current x time that yields 3 a-hours (ah)**. Since the terminal voltage of AA batteries is 1.5 volts, the energy stored in one AA battery is 1.5 v x 3 Ah or 4.5 watt-hours.
So, there you have it; divide energy stored by supergreeniebattery by energy stored by AA battery:
I don't know what meaning we could attach to an infinite number of eights. Let's say that the correct number of AA batteries
is 888,999 and leave it at that.
That's par for the course for greenies, they have a tendency to be over-optimistic (And I'm known for my understatements.)
** Actually this is not true for AAs. They will provide 100 ma (0.1a) for 30 hours, 10 ma (.01 a) for 300 hours and so on. But if you try to get much more than 100 ma continuously from them, they will not yield their 3 ampere-hours.