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Question: How long should a UPS be able to last?

Answer: That question has many answers depending on what the ups supports, how critical it is to avoid shutdowns, and what if any other back up power is also available, like a generator.

Most utility outages have very short duration’s but they are often followed closely by more brief outages and power quality issues that can harm sensitive equipment. Long outages are much less frequent and can last for hours, and even days.

Most companies want to be covered for short interruptions and power problems that often follow close behind an outage, but are not willing to pay for generation equipment so they can withstand a long interruption that might only occur once a year. In order to be safe most companies want battery powered systems that are designed to last 15 to 20 minutes, so they always have enough time to see if utility power stabilizes during the first 5 minutes of an outage. If an outage causes problems for more than 5 minutes they usually shut down their equipment and don’t restart until they are sure utility power is stable again.

In most places 15 minutes of reserve capacity is enough to avoid 90% of the utility interruptions. If you want to survive the longer outages you really need a generator because it’s unlikely you can continue occupying your office if all the AC is off. If you have a server room, it probably can’t run more than about 15 minutes without the AC.

Companies that have generators can get away with less UPS reserve time, and some even use flywheel UPS equipment that can only provide about 20 to 30 seconds of backup power. 10 seconds is enough for most generators to start and take over, they usually switch over when they ramp up to 80% of their capacity.