For most people the short answer is, mains power is so reliable that it’s not worth your bother to worry about the emergency situations. As long as things work, even a significant amount of inefficiency doesn’t add up. So stop worrying, just get things working.

If, on the off chance, you actually are using emergency power a lot, for whatever reason, then yes, this is a worrying about. In that case, you no longer want a short answer. You need a long, complex answer that covers all the different possibilities. So you need to start studying. Be prepared to spend months working at all the details for your specific situation.