Battery runtime calculator
How long will your UPS hold under load?
Enter your UPS rating and battery bank energy. We model runtime across the standard load curve (10% — 100%) for VRLA and lithium chemistries, including inverter and discharge-curve efficiency.
Step 1 · UPS configuration
Nameplate VA rating of your UPS
Output power factor (0.9 default; modern UPS often 1.0)
Total nameplate energy. Multiply Ah × volts (e.g. 24× 7Ah × 12V = 2016 Wh).
Step 2 · Battery chemistry
Estimated runtime by load
| % of full load | Load (W) | Estimated runtime | Application |
|---|---|---|---|
| 10% | 900 | 1 h 55 m | Light load (e.g. comms only) |
| 25% | 2,250 | 46 min | Quarter load (typical idle) |
| 50% | 4,500 | 22 min | Half load (recommended sustained) |
| 75% | 6,750 | 15 min | Three-quarter load (peak) |
| 100% | 9,000 | 11 min | Full load (worst case) |
Runtimes assume an 80% depth-of-discharge cutoff and an inverter efficiency of 92%. Real runtime degrades with battery age (~20% loss across 4-5y for VRLA) and elevated ambient temperature (Arrhenius rule: ~50% capacity loss per 10°C above 25°C). Commission a site discharge test annually.
We deliver a battery sizing report with cell-level discharge data, AS/NZS 5139 compliance, and a documented service-life plan.
Caveats & methodology
Why your real runtime will differ
Battery age. VRLA capacity drops ~20% over a 4-5 year service life. A 5-year-old battery rated for 12 minutes at full load may deliver 7-9 minutes in practice. Annual discharge testing is the only way to know real capacity.
Ambient temperature. The Arrhenius rule: every 10°C above 25°C halves battery service life. A 35°C UPS room burns through batteries twice as fast as a climate-controlled one — and the runtime degrades commensurately.
Load profile. The calculation assumes a constant resistive load. Inrush from motors and PSU startup pulls significantly higher, shortening runtime in real outages where equipment is cycling.
Inverter efficiency. Modern online UPS run 94-97% at >50% load but drop to 85-90% at light load. We use 92% as a conservative average; check your UPS efficiency curve for tighter sizing.