To paint a rough and ready picture, aviation emissions are very heavily weighted towards richer, less populous countries, whereas electricity generation (and particularly fossil fuel generation) is (to a lesser degree) tilted towards where the mass of population is: https://ourworldindata.org/carbon-footprint-flying#:~:text=W... vs https://ourworldindata.org/grapher/carbon-intensity-electric.... Note that the colour axis is a log scale. It's a compounding effect: more people => more energy; poorer => worse emissions and fewer flights per capita.

I thought I must have slipped a power of 10 too somewhere but if I did I can't spot it.

I found some reference to Gatwick using 2.6 billion litres of fuel a year. If I follow the logic above I get circa 8 billion litres. I think most of this is because a Boeing 737 has a 3000nm range fully fueled which they wouldn't be using normally. In fact I suspect it's impossible to take off fully fueled and with a full complement of passengers (it certainly is for lighter aircraft).

Between that and the efficiency difference mentioned elsewhere I think that explains about an order of magnitude. I'm totally willing to accept they'd need a 1GW power station to power Gatwick but 9GW seems high.

That weight constraint cuts both ways though, right? An electric plane charged for a 500 mile flight weighs the same as one charged for a 2000 mile flight, and the max landing weight of (e.g.) a 737 is substantially lower than the max takeoff weight. That means the maximum passenger load of the electric plane can never be as high as one fuelled by an energy source that leaves the plane over the course of the flight. So yes it's more efficient in terms of direct energy use, but it's less efficient in terms of the ratio of work done moving the passengers to work done moving the vehicle, first because you can't stuff as many on, and second because the mass of the vehicle itself doesn't drop over time.

EDIT: unless, of course, you have removable batteries that let you carry less weight for a shorter flight. That might be the only way to make this practical, and would have some other benefits: you could charge them off-site, for instance. It creates a hell of a logistics problem, but no bigger than liquid fuel.

Also, one factor to take into consideration is that the 9GW figure assumes that the refuelling is uniformly distributed throughout the 24 hours. That won't be true, I could believe peak usage being double the average. If that's true, the worst-case 9GW isn't what you need to work to, it's 18GW peak. If we go with the 2.6 billion vs 8 billion L ratio as telling us the true power requirement, that gets us back up to 2.925GW average, 5.85GW peak.

google and wolfram alpha tells me one fully tanked 737 stores 16 tons of kerosene, which translates to 261.1 GJ at 35% efficiency (72.5 MWh). doesn't sound too far off. assuming the same energy will be required for an electric airliner and you want to charge it to full in an hour... you probably need much more than 72.5MW power plant per aircraft because fast charging is nonlinear...? numbers which are hard to comprehend at scale in any case