UPS Size Calculator
Quick answer: A 400 W load at 0.7 power factor needs about 571 VA — choose a 750 VA UPS, the next standard size up.
Find the right UPS (uninterruptible power supply) for your gear. Enter the total watts your devices draw, set the power factor, and get the VA rating you need — plus the next standard UPS size to buy.
Enter your load
Your results
| Required capacity | — |
| Recommended UPS size | — |
Recommendations are estimates for sizing guidance — always check the manufacturer's specs before buying.
How to use this calculator
Add up the wattage of everything you want on battery backup (PC, monitor, router, etc.), enter the total, and adjust the power factor if you know your equipment's rating. The result updates instantly.
How it works
Required VA = watts ÷ power factor. The tool then rounds up to the next standard UPS size (450, 600, 750, 1000, 1500 VA), which builds in headroom. The 0.7 default power factor is an editable assumption — check your equipment specs. Battery runtime is not estimated here since it depends on the specific UPS battery.
Worked example
400 W ÷ 0.7 = 571.4 VA → next standard size = 750 VA.
Reference data
| Standard size | Rating |
|---|---|
| Entry | 450 VA |
| Small desktop | 600 VA |
| Desktop / network | 750 VA |
| Workstation | 1,000 VA |
| Multi-device | 1,500 VA |
Source: standard UPS product lineups (2026).
Method & sources
Apparent power (VA) equals real power (watts) divided by the power factor, so a 400 W load at 0.7 PF draws 571 VA. The recommendation then rounds up to the nearest standard UPS size from the table above, which both matches how UPS units are sold and adds headroom for inrush current and future devices. The 0.7 power factor default is an assumption — many modern PC power supplies run at 0.9 or higher, which would lower the required VA. Battery runtime is intentionally not estimated: it depends on the specific UPS model's battery capacity and the actual load.
- Standard UPS size tiers (2026 product lineups): APC by Schneider Electric — UPS products
- Power factor and VA vs watts relationship: APC support FAQ
Frequently asked questions
What is the difference between watts and VA?
Watts is real power; VA (volt-amperes) is apparent power. The power factor links them: VA = watts ÷ power factor.
Should I oversize the UPS?
Rounding up to the next standard size already adds roughly 20–30% headroom, which is the usual practice.
Does this tell me battery runtime?
No — runtime depends on the specific UPS battery and load. Size for the load first, then check the manufacturer's runtime chart.
What power factor should I enter?
The 0.7 default is an editable assumption. Modern PC power supplies often run 0.9+, so check your specs.