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.