UPS Runtime Calculator
Estimate how long a UPS battery will power a connected load, factoring in system efficiency and usable depth of discharge.
Inputs
Total rated battery energy in watt-hours
DC-to-AC inverter conversion efficiency, typically 85-95%
How much of rated capacity is safe to draw down — lead-acid ~50%, lithium ~80-100%
Estimated Runtime
172.8min
Estimated Runtime
2.88hours
Usable Energy (after efficiency + DoD)
1,440.0Wh
Energy Not Usable (losses + reserve)
560.0Wh
Step by step
Values used
Battery Capacity = 2,000 Wh; System Efficiency = 90; Usable Depth of Discharge (DoD) = 80; Connected Load = 500 W
Runtime in minutes
runtime_min = (battery_Wh × efficiency × DoD) / load_watts × 60
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
How it works
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
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.