How is UPS battery runtime calculated?
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. See also our Download Time Calculator.
What is the difference between Wh and Ah on a UPS battery?
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.
Why does UPS efficiency affect runtime?
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.
What is Depth of Discharge (DoD) and why does it matter?
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. You might also find our calculate Data Transfer Time useful.
How do I find the wattage of my connected devices?
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.
What is the difference between Watts and VA for UPS load?
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.
Can I extend my UPS runtime?
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.
How accurate is this UPS runtime calculator?
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.