UPS Runtime
Calculate how long a UPS battery will sustain a given load before depletion.
About UPS Runtime
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.
Formula
Runtime in minutes
runtime_min = (battery_Wh × efficiency × DoD) / load_watts × 60
- E_{Wh}
- Battery rated capacity in watt-hours
- \eta
- System/inverter efficiency (0-1)
- D
- Usable depth of discharge (0-1)
- P_{load}
- Connected load in watts
Worked example
With the default values below, UPS Runtime returns Estimated Runtime of 172.8. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Battery Capacityminimum 1 Wh
- 2,000 Wh
- System Efficiencyaccepts 1–100
- 90
- Usable Depth of Discharge (DoD)accepts 1–100
- 80
- Connected Loadminimum 1 W
- 500 W
Results
- Estimated Runtime
- 172.8 min
- Estimated Runtime
- 2.88 hours
- Usable Energy (after efficiency + DoD)
- 1,440.0 Wh
- Energy Not Usable (losses + reserve)
- 560.0 Wh
Step by step
- 1Values used
- 2Runtime in minutes
- 3Estimated Runtime= 172.8 min
- 4Estimated Runtime= 2.88 hours
- 5Usable Energy (after efficiency + DoD)= 1,440.0 Wh
- 6Energy Not Usable (losses + reserve)= 560.0 Wh
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What is depth of discharge (DoD) and why does it matter?
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.
Why isn't runtime just capacity divided by load?
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.
How can I extend UPS runtime?
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.