You joke but make a good point, most major installations have backup diesel powered generators to provide power for a certain limited but generous time.

What happened with Heathrows generators? Did they kick in? Do they have any?

Edits: yes they did kick in as normal https://www.standard.co.uk/news/uk/heathrow-ed-miliband-nati...

Note that usually backup generators are only for essentials like air traffic control, landing lights, and operating emergency stuff.

It would be too unsightly to put solar panels on those large flat shed roofs.