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There is an interesting paper published by Microsoft and researchers from Penn State. Lless than 5% of all outages last longer than 240 minutes, while outages of 30 minutes or less account for over 70% of occurrences. So, for the majority of events, a diesel generator is not needed and cheaper UPCs will do the job. The problem is underprovisioning based on the 70% and not being prepared for more lengthy outages. There isn’t a single figure I can give for how long backup power should last because there are many different data centers with different needs and budgets. I suggest reading the paper to gain some additional insight into how to optimize backup power infrastructure.