UPS Sizing Calculator
Find the correct UPS VA rating for your connected equipment.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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A UPS (uninterruptible power supply) is rated in volt-amperes (VA), not watts, because it must handle both real and reactive power. This calculator converts your watt load to the VA rating you need.
- Total load in watts. Add up the wattage of all devices the UPS will protect. Check labels or product specifications. For computers, use the PSU rating; for servers, check the nameplate.
- Power factor. Most modern computers and servers have a power factor between 0.95 and 1.0 (power factor corrected). Older equipment, motors, and lighting may be 0.6 to 0.8. When unsure, use 0.7 (conservative).
- Desired runtime. This is informational — actual runtime depends on the specific UPS battery, which manufacturers publish in tables. The calculator shows the VA capacity needed, not runtime directly.
- Read the result. The recommended VA adds a 25% safety margin (standard industry practice) above the mathematical minimum.
About UPS Sizing
The formula is: VA required = watts / power factor. A 500 W load with a 0.7 power factor needs 500 / 0.7 = 714 VA minimum. With the 25% safety margin, buy a UPS rated at least 893 VA. Round up to the nearest standard size (typically 900 VA, 1000 VA, or 1500 VA).
Manufacturers typically rate runtime at 50% load or full load, so a 1000 VA UPS does not give twice as long as a 500 VA at the same load. Always consult the manufacturer's runtime chart for your specific load, or use a UPS with a runtime calculator on their website.
Frequently Asked Questions
What is the difference between watts and VA on a UPS?
Watts measure real power (energy actually consumed). VA (volt-amperes) measure apparent power, which includes both real power and reactive power drawn by inductive or capacitive loads. For purely resistive loads, watts equal VA. For most computer equipment, VA is roughly watts divided by the power factor (typically 0.7 to 0.9). A UPS must handle the full apparent power (VA) even though only the real power (watts) does actual work.
How many watts does a typical computer draw?
A desktop PC under typical use draws 100 to 300 W depending on the CPU and GPU. A gaming PC with a high-end GPU can draw 400 to 800 W. Laptops typically draw 20 to 60 W. Servers range from 100 W for small network-attached storage to over 1000 W for high-density rack servers. Use a smart plug with power monitoring for an accurate measurement of your specific hardware.
How long will a UPS last during a power outage?
Runtime depends on the UPS battery capacity and the connected load. Most consumer UPS units provide 5 to 20 minutes of runtime at full load — enough to save work and shut down gracefully. Higher-capacity units and extended battery modules can provide 30 minutes to several hours. Check the manufacturer's runtime chart for your specific UPS model and actual watt load.
Should I connect everything to a UPS or just critical devices?
Connect only critical devices that need to stay on during an outage: computers, network equipment, and external drives. Printers, monitors, and desk lamps can connect to the UPS's surge-protected outlets (if equipped) but should not draw from the battery. Reducing the battery load maximizes runtime for the essential equipment. Only count battery-backed devices in this calculator.