UPS battery runtime calculator
Estimate how long your UPS batteries will keep network and server equipment running.
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Enter the load in watts (or VA and power factor), the battery voltage, amp-hour rating and number of batteries. The calculator estimates runtime after allowing for depth of discharge and inverter efficiency, so you can size batteries for racks, network closets and CCTV recorders.
The formula
Runtime (hours) = battery voltage × amp-hours × number of batteries × depth of discharge × inverter efficiency ÷ load in watts. Two 12 V 100 Ah batteries hold 2,400 Wh; at 80% depth of discharge and 90% efficiency that gives about 1,730 Wh, or just under three hours at 600 W.
Why real runtime is shorter
Lead-acid batteries deliver less of their rated capacity at high discharge rates (Peukert's effect), lose capacity as they age and perform worse in heat. For SMF/VRLA batteries, plan to replace them every 3–5 years and add 20–30% to the battery bank for ageing. Lithium batteries keep their capacity better at high loads.
Watts and VA
UPS ratings are often given in VA. Real power in watts is VA × power factor; most modern IT equipment has a power factor of 0.9–1.0, and older UPS units are rated at 0.6–0.8.
Common questions
How long will a 1 kVA UPS last?
It depends on the batteries, not the kVA rating. A 1 kVA UPS with two 12 V 7 Ah batteries runs a 300 W load for roughly 15–20 minutes.
What depth of discharge should I use?
80% for lithium and 50–80% for lead-acid. Discharging lead-acid batteries deeply and often shortens their life.
Should we use one battery bank for everything?
Separate critical loads such as core switches and firewalls from non-critical loads, or use load shedding, so the essentials keep running longest.
