Hello. Hope you are doing well. Could you please explain the following questions?

‘[If there were] a power failure, the hospital will run on electricity from its own generators, which can operate for 200 hours.’

[If there were] is incorrect and should be replaced by [Should there be]. Can you please explain why?

This is one of those conditional tense issues that I must admit I have a hard time explaining other than to tell you what’s right and what isn’t. :/

You’re constrained by “the hospital will,” which cannot be changed. “Should there be” goes with that. If you wanted to use “if there were” in the beginning of the sentence, you’d need to have “the hospital would” later on.