Enter your UPS battery capacity (Wh or Ah + voltage), connected load (watts), and UPS efficiency to find out exactly how long your UPS will keep your devices running during a power outage. Results include estimated runtime, usable battery energy, and a visual breakdown of load vs. capacity.
Estimated Runtime
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Runtime in Hours
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Usable Battery Energy
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Effective Load (after efficiency)
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Load as % of Battery Capacity
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Runtime is calculated by dividing the usable battery energy (in Watt-hours) by the effective load (in Watts, adjusted for UPS efficiency). The formula is: Runtime (hours) = (Battery Wh × Depth of Discharge) / (Load W / UPS Efficiency). This gives you the approximate time the UPS can sustain your connected devices.
Watt-hours (Wh) measure total energy stored, while Amp-hours (Ah) measure charge capacity at a given voltage. You can convert: Wh = Ah × Voltage. For example, a 9Ah battery at 12V equals 108Wh. Most UPS spec sheets list both, but Wh is the most direct unit for runtime calculations.
A UPS is not 100% efficient — it consumes some energy during the conversion and regulation process. If your UPS is 90% efficient, only 90% of the drawn battery power actually reaches your devices. This means the effective load the battery must support is slightly higher than your device's rated wattage, reducing overall runtime.
Depth of Discharge is the percentage of battery capacity you can safely use before recharging. Discharging a lead-acid battery below 80% DoD regularly shortens its lifespan significantly. Lithium-ion batteries can typically handle 90–95% DoD. Setting the correct DoD gives you a realistic — and battery-safe — runtime estimate.
Check the label on the back or bottom of each device for a Watts (W) rating. If only VA or amps are listed, multiply amps by voltage to get watts (e.g. 2A × 120V = 240W). For the most accurate measurement, use a plug-in power meter (such as a Kill-A-Watt) that shows real-time consumption under actual operating conditions.
Watts (W) represent real power consumed, while Volt-Amperes (VA) represent apparent power. The ratio between them is the power factor. Most modern electronic equipment has a power factor between 0.6 and 0.9. UPS capacity is often rated in VA, but runtime is governed by the actual Watt load. Always convert VA to Watts using your equipment's power factor for accurate results.
Yes. You can extend runtime by reducing the connected load (unplug non-essential devices), adding external battery packs if your UPS model supports them, upgrading to a higher-capacity battery, or choosing a lithium-ion UPS which offers higher energy density. Even small load reductions can meaningfully increase runtime — cutting load in half roughly doubles runtime.
This calculator provides a reliable theoretical estimate based on your inputs. Real-world runtime can vary due to battery age and health, temperature, load fluctuations, and the battery's discharge curve at different load levels. Manufacturers' published runtime charts are typically measured under controlled conditions with a new, fully charged battery.