UPS Size Calculator
Enter the total watts of the equipment you want to protect, the UPS power factor, how much headroom you want and the runtime. The calculator gives the minimum VA rating, the nearest standard UPS and the battery energy for that runtime.
- Watts to VA
- Standard UPS sizes
- Battery for runtime
Add up the watts on each device's label or power supply. A gaming PC and two monitors is often 400 to 700 W.
A UPS is rated in VA, and your equipment draws watts. Divide watts by the UPS power factor to get VA, then add 20% to 30% headroom so the UPS is not running flat out. A 600 W computer setup needs about an 850 VA UPS.
UPS size chart
Power factor 0.9, 25% headroom. Search for a load.
| Load (W) | Minimum VA | Standard UPS | 10 min battery |
|---|---|---|---|
| 200 | 278 | 600 VA | 37 Wh |
| 400 | 556 | 600 VA | 74 Wh |
| 600 | 833 | 850 VA | 111 Wh |
| 800 | 1111 | 1500 VA | 148 Wh |
| 1000 | 1389 | 1500 VA | 185 Wh |
| 1500 | 2083 | 2200 VA | 278 Wh |
| 2000 | 2778 | 3000 VA | 370 Wh |
Battery energy is for 10 minutes at full load, allowing 90% inverter efficiency.
Load and UPS size
Power factor 0.9. Slide the load.
The gauge under the drawing shows how hard the chosen UPS is working. Keep it under about 80%.
How to size a UPS
Watts, VA, headroom. Step through it.
List the devices
PC, monitors, router.
Laser printers and heaters have big start-up surges. Keep them off the UPS battery outlets.
Standby, line-interactive or online
UPS types. Compare them.
Switches to battery when power fails.
Corrects low and high voltage without battery.
Always runs from the inverter.
Online UPS units rebuild the power all the time, so they suit servers and sensitive kit.
Before buying
UPS checks. Tick each item.
Tick each item as you go.
Batteries usually last 3 to 5 years. Check the replacement cost before you buy.
What you get
UPS sizing. Each tile explains one result.
Load ÷ power factor.
Next size up.
On that UPS.
For your runtime.
At its voltage.
Real runtime figures from the maker's chart are more accurate than any estimate, as battery output drops at high current.
Frequently Asked Questions
No question matches that search.
What size UPS do I need for my PC?
Add up the watts and divide by 0.9, then add about 25%. A 500 W PC needs about a 700 VA UPS, so an 850 VA unit.
What is the difference between VA and watts?
VA is apparent power. Watts is real power. Watts = VA × power factor.
How long will a UPS last?
Divide the battery Wh by the load in watts and allow about 90% efficiency.
Is a bigger UPS better?
It runs longer and cooler at the same load, but costs more.
What power factor do UPS units have?
Most new units are 0.9. Older or budget ones are 0.6.
Can I plug a laser printer into a UPS?
Only into a surge-only outlet, as its warm-up draws a big surge.
How do I convert VA to watts?
Multiply VA by the power factor.
Power that stays on
Add up your watts, convert to VA and allow headroom. The right UPS keeps your work safe when the lights go out.
More size calculators
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