Not only will a data center of a generator, they will test it regularly and if it doesn't work get it fixed.

The power company has a long list of who has backup power. I know of one factory where the generator was installed in the 1920s on a boiler from the 1880's - it is horribly inefficient, but the power company still gives the owners incentive to keep it working because for 4x the normal cost of power and 12 hours notice that generator can run the entire town it is in, which they do every 5 years when things really go wrong with the grid.