UPS Runtime Calculator
Estimate how long your UPS will keep your equipment running during a power outage.
Enter UPS VA rating and total load watts.
UPS (Uninterruptible Power Supply) runtime estimates how long a UPS battery can power connected equipment during an outage. This depends on the battery capacity, inverter efficiency, and total load drawn.
Runtime Formula:
Runtime (hours) = (Battery Capacity (Wh) × Efficiency) / Load (Watts)
Or, if the UPS is rated in VA and the battery in amp-hours (Ah):
Battery Wh = Battery Voltage × Ah UPS Watt Capacity = VA Rating × Power Factor (typically 0.6)
Worked example: UPS: 1500 VA, 24V battery bank, 9 Ah battery Battery Wh = 24 × 9 = 216 Wh UPS inverter efficiency: typically 85–90% → use 0.85 Connected load: 200 W (desktop PC + monitor + router)
Runtime = (216 × 0.85) / 200 = 183.6 / 200 = 0.918 hours ≈ 55 minutes
Common UPS load ratings:
| Equipment | Typical Load |
|---|---|
| Desktop computer | 150–300 W |
| Gaming PC | 300–600 W |
| 27-inch monitor | 25–40 W |
| Home router | 10–20 W |
| Network switch (8-port) | 10–15 W |
| NAS drive (2-bay) | 20–40 W |
| Small server | 200–400 W |
Practical factors that reduce runtime:
- Aging batteries lose 30–50% capacity after 3–4 years
- High temperature accelerates capacity loss
- Running near maximum VA rating stresses the battery and lowers efficiency
Load percentage matters: A UPS at 50% load runs more efficiently than at 90% load. For maximum runtime, add more battery capacity rather than oversizing individual UPS units.
Sizing recommendation: Choose a UPS whose watt rating is at least 1.5× your total load, leaving headroom for efficiency loss and battery aging.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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