Calculate how long a UPS battery will sustain a given load before depletion.
UPS runtime depends on how much of the battery's rated energy is actually deliverable to the load. Two factors shrink that usable pool below the nameplate rating: system efficiency (DC-to-AC inverter conversion losses) and depth of discharge (the safe fraction of capacity you can draw without damaging the battery or triggering a low-voltage cutoff). Multiplying rated capacity by both factors gives usable energy, which is then divided by the load and converted to minutes.
Runtime in minutes
runtime_min = (battery_Wh × efficiency × DoD) / load_watts × 60
DoD is the fraction of a battery's rated capacity that can be safely drawn without excessive wear or damage. Lead-acid batteries are typically limited to ~50% DoD for reasonable cycle life, while lithium-ion (e.g. LiFePO4) batteries tolerate 80-100% DoD — using the full nameplate capacity for a lead-acid battery overstates real-world runtime significantly.
Because two layers of loss sit between rated capacity and usable output: inverter/system efficiency (DC-to-AC conversion losses, typically 5-15%) and depth-of-discharge limits (the safe usable fraction of the battery). Both must be applied before dividing by load to get a realistic runtime estimate.
Add external battery packs to increase rated capacity, choose a battery chemistry with higher usable DoD (e.g. lithium over lead-acid), select a more efficient inverter, or reduce the connected load by shedding non-critical equipment during an outage.