Battery Backup Runtime Calculator
Estimate UPS runtime from load and battery capacity. Lithium and VRLA chemistries.
Key takeaways
- Runtime falls non-linearly with load - doubling load typically cuts runtime by more than half (Peukert effect).
- VRLA loses 20-30% of nameplate capacity at 80% depth of discharge - what's usable is less than what's labeled.
- Lithium-ion holds rated capacity much better at high discharge rates - the working choice for short, high-load runtimes.
- Temperature matters: VRLA loses ~50% capacity at 0 deg C and ages 2x faster above 25 deg C.
Inputs
Results
How this works
- Battery energy (Wh) = Volts × Amp-hours. A 48V / 65 Ah battery stores 3120 Wh nominal.
- Usable energy = nominal × depth of discharge × temperature derate. VRLA at 80% DoD in a normal-temp room uses 80% of nameplate; at 0 deg C, only ~40%.
- Energy at the load = usable energy × UPS efficiency. The inverter eats 5-10% converting DC battery to AC output.
- Runtime = energy at load (Wh) / load (W) × 60 minutes.
Worked example
A 24-port PoE+ switch + 16 cameras + 1U NVR + monitor = 1000W IT load. UPS has a 48V / 65 Ah VRLA battery pack at 80% DoD, 92% inverter efficiency, normal room temperature:
- Stored: 48 × 65 = 3120 Wh
- Usable: 3120 × 0.80 × 1.0 = 2496 Wh
- At load: 2496 × 0.92 = 2296 Wh
- Runtime: 2296 / 1000 = 2.3 hours = ~138 minutes
This is more than enough for graceful shutdown or for a generator with a 5-minute transfer window. For a 30-min target with this load, you'd need ~600 Wh usable - a smaller battery pack saves cost.
VRLA vs lithium quick reference
| Attribute | VRLA (sealed lead-acid) | Lithium-ion (LFP) |
|---|---|---|
| Energy density | Low (~35 Wh/kg) | High (~120 Wh/kg) |
| Cycle life (80% DoD) | 200-500 cycles | 3000-6000 cycles |
| Calendar life | 3-5 years (cool), 18-30 mo (warm) | 8-12 years |
| Temperature tolerance | Narrow (best 20-25 deg C) | Wide (-20 to +60 deg C) |
| Peukert effect | Significant (10-20% loss at high rate) | Minimal |
| Self-discharge | 3-5% per month | 1-2% per month |
| Cost per kWh stored | $120-200 | $300-500 |
| 10-year TCO (cycle + replace) | $300-450 per kWh | $300-500 per kWh |
| Recyclability | ~97% (mature stream) | ~50% (improving) |
FAQ
Why is runtime less than the simple kWh / kW math?
Three reasons. First, you can't fully discharge a battery - the UPS cuts off at 80-90% DoD to protect the cells. Second, the inverter loses 5-10% converting DC to AC. Third, VRLA capacity drops at high discharge rates (Peukert).
Should I use lithium or lead-acid?
For runtimes under 10 min at less than 5 kW, VRLA is cheaper. For longer runtimes, higher loads, hot environments, or critical applications, lithium pays back in 5-7 years through longer life and smaller footprint.
How accurate is this calculator?
Within 15-20% for typical UPS deployments. The biggest unknowns are battery age (a 3-year-old battery delivers ~70% of nameplate) and ambient temperature swings. For mission-critical sizing, derate by an extra 25% or specify lithium.
Can I add external battery modules to any UPS?
Only if the UPS is rated for them - check for "EBM" (external battery module) support. Tower and rack online UPS from Vertiv, APC, Eaton, Tripp Lite often support 1-4 EBMs daisy-chained. Standby UPS typically does not.
What's the lifespan I should plan for?
VRLA: replace at 3-4 years in a cool room, 18-30 months in a warm room. Lithium: 8-12 years. Use the UPS's battery monitoring to track impedance - rising impedance is the early warning.