Core definitions
| Term | Use | Common mistake |
|---|---|---|
| Watts (W) | Real power consumed by connected equipment | Using power-supply rating as measured load |
| Volt-amperes (VA) | Apparent-power capacity | Treating VA as battery energy |
| Power factor | W ÷ VA relationship for a load | Assuming 1.0 without documentation |
| Watt-hours (Wh) | Approximate energy | Ignoring conversion loss and nonlinear discharge |
Planning equations
VA floor = load watts ÷ power factor. Apply documented headroom to both the watt and VA requirements. A simple energy screen is Wh ≈ watts × hours ÷ efficiency, but the final runtime must come from the manufacturer load/runtime curve.
Worked example
A 120 W protected load with 30% headroom becomes 156 W. At power factor 0.7, the apparent-power floor is about 223 VA. A 15-minute energy screen at 80% efficiency is roughly 49 Wh, before model-specific battery behavior.
Limitations
Battery age, temperature, inverter design, load shape, startup behavior, and vendor firmware change runtime. Confirm communication and safe-shutdown compatibility as well as capacity.
Apply the model in UPS Size & Runtime Calculator.