What does it mean, hospitals are running on generators?

Hi, as a non-native English speaker, I got curious when I was "studying" English through this CSI series. There's this situation that a bomb attempt took a place at a hospital and the CSI took a possible (seriously injured) suspect in the medical examiner’s office where they're examining a live... show more Hi, as a non-native English speaker, I got curious when I was "studying" English through this CSI series.

There's this situation that a bomb attempt took a place at a hospital and the CSI took a possible (seriously injured) suspect in the medical examiner’s office where they're examining a live one (almost dying), then a cop comes and asks why they brought a live one there so the examiner says, Hospitals are running on generators and not doing any elective tests so hr brought the live one over.

Here, what I wanna know is,
Does running on generators mean the hospitals are literally using generators to produce electricity, or does it mean something else? Like... due to the bomb attack, they are not taking any serious patients or something? Please help. Thank you!
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