What is UPS runtime and why does it matter?
UPS runtime is the amount of time an uninterruptible power supply can keep connected equipment running during a utility power outage. It's not a fixed number — it depends on the total load, battery capacity, battery chemistry, UPS efficiency, and how deeply the battery is allowed to discharge.
For IT infrastructure, runtime is a critical design parameter. A server rack drawing 2,000 W on a 10 kVA UPS with 9 Ah batteries may get only 8–12 minutes at full load. Reducing that load to 800 W can extend runtime to 25–35 minutes on the same hardware.
This calculator models that relationship so you can size batteries, plan graceful shutdown windows, and compare UPS configurations before you buy.
Key variables
- •Battery capacity (Ah) — charge storage rating at a given discharge rate.
- •Battery voltage (V) — nominal DC bus voltage of the battery string.
- •UPS efficiency (%) — DC-to-AC conversion losses, typically 85–95%.
- •Load (W) — real power drawn by connected equipment.
- •Power factor — ratio of real power (W) to apparent power (VA).
- •Depth of discharge (DoD) — how much battery capacity is usable before cutoff.
UPS runtime formula
Runtime (hours) = (Battery Ah × Battery V × UPS Efficiency × DoD) ÷ (Load W ÷ Power Factor)
Note: This formula gives a theoretical estimate. Real-world runtime is lower — see below.
Worked example
Inputs
- Battery: 100 Ah @ 48 V
- Efficiency: 90%
- Power Factor: 0.9
- Load: 800 W
- DoD: 80% (LiFePO₄)
Calculation
- Energy = 100 × 48 = 4,800 Wh
- Usable = 4,800 × 0.90 × 0.80 = 3,456 Wh
- Effective load = 800 ÷ 0.9 = 889 W
- Runtime = 3,456 ÷ 889 ≈ 3.89 h
Result
3.89 h
≈ 3 h 53 min theoretical
Real-world: expect 3 h 15 min – 3 h 30 min after Peukert and temperature derating.
Battery chemistry comparison
| Chemistry | Usable DoD | Cycle Life | Weight | Best For |
|---|---|---|---|---|
| Sealed Lead-Acid (SLA) | 50% | 200–300 | Heavy | Cost-sensitive, short backup |
| AGM / Gel | 50–60% | 300–500 | Heavy | Sealed, low maintenance |
| LiFePO₄ (LFP) | 80–90% | 2,000–5,000 | Light | Long runtime, frequent cycling |
| NMC / Li-ion | 80–90% | 500–1,500 | Lightest | Portable, compact UPS |
Why real UPS runtime differs from theoretical calculations
Peukert effect
Lead-acid batteries deliver less capacity at high discharge rates. A 100 Ah battery might only provide 60 Ah at 50 A draw.
Temperature
Capacity drops roughly 1% per °C below 25°C. A cold server room can cut runtime by 10–20%.
Battery age
VRLA batteries lose 20% capacity after 3–4 years. A 5-year-old battery may deliver only half its rated Ah.
Inverter efficiency curve
UPS efficiency is highest at 40–70% load. Light loads (below 20%) can drop efficiency to 70–80%.
Power factor mismatch
IT loads often have PF < 1.0. A UPS rated in VA but loaded in W will behave differently than the nameplate suggests.
Cutoff voltage
UPS units stop discharging at a set voltage to protect batteries. This alone can leave 10–20% capacity unused.
Frequently asked questions
How do I calculate UPS runtime for my server rack? ▾
Add up the real wattage of all connected equipment (servers, switches, storage). Divide total watts by the power factor to get effective load. Then apply the formula: Runtime = (Ah × V × Efficiency × DoD) ÷ Effective Load. For mixed loads, use the weighted average power factor.
What is a good UPS efficiency for runtime calculations? ▾
Online double-conversion UPS: 88–94%. Line-interactive: 95–98% in normal mode, but 85–90% on battery. Standby/offline: 90–95%. Use 0.90 as a conservative default for online UPS runtime estimates.
Does power factor affect UPS runtime? ▾
Yes. A load with PF = 0.7 draws 1,000 VA but only 700 W of real power. The UPS inverter must supply the full VA, but battery energy is drawn based on real power. Always divide watts by power factor to get the correct load for runtime math.
Why does my UPS runtime drop so fast as load increases? ▾
Runtime is inversely proportional to load — double the load, halve the runtime. But battery internal resistance and Peukert losses make high-load runtime even shorter than linear. That's why a 100 Ah battery at 20 A might last 4.5 h, but at 40 A only 2 h instead of 2.25 h.
How much runtime do I need for safe server shutdown? ▾
Plan for 10–15 minutes at full load for graceful OS shutdown, virtual machine migration, and storage flush. If you need extended runtime (30+ min), consider a generator interlock or larger battery bank rather than a bigger UPS.