Add up the watts of what goes on the UPS and multiply by 1.25. That keeps the load under 80% of the UPS’s capacity, as CyberPower recommends.
CyberPower’s 125% rule
The smallest UPS for each total load, by CyberPower’s rule
Total load
UPS of at least
100 W
125 W
200 W
250 W
300 W
375 W
400 W
500 W
600 W
750 W
Watts and VA
A UPS carries two ratings, watts and VA, and Schneider Electric (APC) says the equipment plugged in must exceed neither. Check your total against the watt rating, the smaller of the two.
Finding the watts
CyberPower gives three places to look: the device’s user manual, the safety label on the device or its power supply, or volts times amps.
How long it runs
CyberPower says most power outages last only five minutes, though some run longer. It suggests checking how long the UPS runs at full load. For more time, it suggests a UPS with more capacity or added battery modules.
No, says Schneider Electric (APC): for computers and UPSs, the VA rating is always equal to or larger than the watt rating. The ratio of the two is the power factor.
From CyberPower’s runtime advice and Schneider Electric’s (APC) note on VA and watts, read on October 25, 2026. Checked against published data from Schneider Electric (APC) and CyberPower.
See all 2 sources
Schneider Electric blog, VA vs watts: What's the difference anyway?
CyberPower blog, Runtime Recommendations
Battery sizes come from battery makers' datasheets and device makers' own manuals and help pages. Check your device's manual or the old battery before you buy. If anyone may have swallowed a battery, call the National Battery Ingestion Hotline at 1-800-498-8666 or your poison center at 1-800-222-1222 straight away.