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ITFrequently Asked Questions · UPS
What power rating should my UPS have?
Choosing a UPS with the right power rating takes two simple steps: 1. Check the power rating on the label of each device you want to protect/back up, find its power in VA, and add up the power of all the devices together. If the power is given in watts or amperes, you can easily convert it to VA with these formulas: VA = 1.43 x W (when watts are given), VA = 230 x A (when amperes are given). 2. Choose a UPS with a power rating at least 30% higher than the load you need to back up, to ensure reliable protection.
The calculation is simple, but three points make the difference. First, a UPS has a limit in both VA and W. The 1.43 factor corresponds to a power factor of 0.7 and is a conservative estimate; modern computer power supplies have a power factor close to 1. So check that the total in watts is also below the UPS's watt rating, not just the VA.
Second, a device's label shows the maximum rating of its power supply, not its actual consumption. A desktop computer with a 500 W power supply usually draws much less in everyday use. If you want accuracy, measure with a simple plug-in power meter.
Third, the power rating determines how much load the UPS can carry, while runtime depends on the batteries. If you need more minutes, you may need a larger UPS or a model with battery expansion, even if the power rating is sufficient.
The 30% margin covers start-up peaks, battery ageing and future additions. A common mistake is counting loads that do not need backup, such as laser printers, heaters or speakers, which overload the UPS for no reason.
