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The way to calculate the approximate power life (or length of time the UPS power supply will keep things running) on power outage, is to take the Watt-hour capacity figure for the UPS backup, then divide the amount of Watts demand (or load) that will be placed on it, and get the number of hours that way.
For example: you are examining a UPS backup with 400 Watt-hours capacity.
And your server, which will be the only load on it upon power failure, requires 400 Watts of power, average, continuous, or expectable load.
400 W-h / 400 W = 1 (that is, one hour, before the UPS signals shutdown to the server, if it has that signaling capability before the battery goes dead.)
(Actually, since this sort of thing is in practice less than perfect, the expectable UPS power-supplied-after-power-failure time would be more like 45 or 50 minutes, rather than a full hour.)
Let me know if you need more information, or for me to advise you as to specific details on any given example.
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