How long will a 40 watt light bulb run on a UPS? This requires a bit of understanding of the proper measures of a UPS capacity.
First of all, Watts and Volt-Amps are not the same! Volt-Amps is a measure of the “apparent load” of a system. The “power factor” is the ratio of actual power consumption to apparent load, which is typically less than one (60% to 85%) for small computers like desktop PCs due to them behaving like “capacitive loads.” Purely resistive loads such as incandescent light bulbs and electric heaters will have a power factor of 1.
So, when a UPS specifies its power capacity in terms of Volt-Amps, you should generally multiply that by 1 / 60% = 1.67 to obtain the actual power rating in Watts that is guaranteed to be available.
To be on the safe side, you can always assume your connected equipment has a power factor of 1 and your UPS power rating in Watts is 60% of the Volt-Amp rating. This will result in an oversized UPS that runs for longer than calculated.
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