UPS Calculator - Watts, VA & Battery Runtime
Add a PC, router and other loads to estimate UPS watts, VA and battery energy for the backup time you need.
Equipment on battery
Example loads only. Replace watts with measured draw or the equipment label before buying a UPS.
Sizing assumptions
Your UPS sizing
Formulas and limits
- Load W = Σ(device W × quantity). Load VA = load W ÷ equipment power factor. Minimum UPS ratings = load W and VA × (1 + capacity margin ÷ 100). Verify both ratings on the exact UPS model.
- Nominal battery Wh for target = load W × target minutes ÷ 60 ÷ usable-energy fraction. Idealized runtime = installed battery Wh × usable-energy fraction ÷ load W × 60.
Runtime is a planning approximation, not a guarantee. Battery discharge is nonlinear; inverter efficiency, battery age, temperature, startup surges and auto-shutdown change real time. Compare the exact UPS manufacturer's runtime curve at your measured load. Do not infer runtime from VA alone.
How to size a UPS for a PC, router or server
- Add every device that must stay powered: computer, monitor, router, switch, modem or server. Enter real operating watts and quantity; the initial values are examples, not recommendations.
- Set the equipment's effective power factor and a capacity margin. The calculator shows the load in W and VA and the minimum W and VA ratings to check on a candidate UPS.
- Choose the backup time and the share of nominal battery energy you expect to be usable. Optionally enter the UPS battery energy in Wh for an idealized runtime estimate.
- Review the formulas, then check the chosen model's official runtime chart at the measured load. The calculator does not account for inrush current, outlet count, waveform compatibility or battery discharge curves.
UPS sizing questions
Is a 1500 VA UPS always enough for a 1000 W computer?
No. A UPS has separate VA and W output limits. The model must exceed both calculated ratings at the planned load.
Can I calculate exact runtime from VA?
No. VA describes power capacity, not stored battery energy. Even Wh-based estimates need the model's load-versus-runtime chart.
Should I use the PSU's advertised watts?
Use measured wall power under the workload you need to support. A PSU's nameplate is its output capacity, not necessarily the computer's actual draw.