The time a UPS can support its connected load from batteries after utility power is lost. It depends on stored battery energy, depth of discharge, UPS efficiency, power factor, and load.

UPS Runtime Calculator

Estimate battery backup duration from DC voltage, amp-hour capacity, depth of discharge, UPS efficiency, power factor, and connected load. Useful for generator ride-through and graceful-shutdown planning.

Typical DoD

80%

Typical UPS efficiency

95%

Common target

5–15min

Bus voltage

480VDC

Load input Toggles load-unit interpretation

Battery system voltage (V) Nominal DC bus voltage

Total battery capacity (Ah) Sum across parallel strings

UPS efficiency (dec) Inverter conversion efficiency, e.g. 0.95

Inverter power factor (PF) Used if load entered as kVA

Load (kW) Critical load served by the UPS

Depth of discharge (%) Typically 80% for planning margin

Runtime

109.5min

01:49 hh:mm

Battery energy available

76.8kWh

Effective load

42.1kW

Clock time

01:49

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Tip At high discharge rates, batteries deliver less than nameplate Ah (Peukert effect). Compare this estimate with the vendor's discharge curve at your actual kW level before finalizing autonomy.

Worked example

480 VDC battery bus, 200 Ah total, 80% DoD, 95% UPS efficiency, 40 kW critical load on a unity-PF inverter.

1. Battery energy E = (V × Ah × DoD) / 1000 E = (480 × 200 × 0.8) / 1000 = 76.8 kWh 2. Effective load (inc. UPS losses) Eff_load = RealLoad / η Eff_load = 40 / 0.95 = 42.1 kW 3. Runtime t = 76.8 / 42.1 = 1.824 hr ≈ 109.5 min (01:49)

Warn Peukert's law correction. The estimated runtime assumes the battery's nameplate Ah is delivered at the design discharge rate. For sub-15-minute runtimes (typical UPS sizing), Peukert's law applies — actual capacity is significantly lower than the 20-hour Ah rating.

      Peukert: t = C × (C/I)(k − 1) / I,  with k > 1
    

Battery chemistry	Peukert k	Capacity at 5-min load vs 20-hr rating
VRLA (sealed lead-acid)	1.20 – 1.40	~ 60 – 70 % delivered
AGM (telecom-grade)	1.15 – 1.25	~ 70 – 80 % delivered
Li-ion (LFP / NMC)	1.02 – 1.05	~ 95 – 99 % delivered

For a 5-minute runtime estimate from a 20-hour Ah on VRLA, multiply calculated runtime by ~ 0.65. Li-ion needs no Peukert correction at typical UPS discharge rates.

Tip For a defensible runtime sizing on lead-acid, follow IEEE 485 (vented) or IEEE 1184 (VRLA stationary) — both include cell-aging factors and discharge-curve corrections beyond simple Ah math. Manufacturer discharge curves (e.g. C&D Technologies, EnerSys datasheets) override these estimates for the specific cell at the specific load.

Typical UPS battery configurations

UPS size	String voltage	Ah range	Runtime @ full load
10 kVA	192 – 240 VDC	26 – 65 Ah	5 – 20 min
20 kVA	240 – 384 VDC	40 – 100 Ah	5 – 15 min
40 kVA	384 – 480 VDC	50 – 150 Ah	5 – 15 min
80 kVA	480 VDC	75 – 200 Ah	5 – 12 min
160 kVA	480 – 540 VDC	100 – 300 Ah	5 – 10 min
300+ kVA	480 – 540 VDC	150 – 600+ Ah	3 – 10 min

Common mistakes

Ignoring UPS efficiency. A 95% inverter adds 5% to the effective battery load — small but material at short runtimes.
Using nameplate Ah at high C-rate. Battery Ah is typically specified at an 8- or 20-hour rate; at 10-minute discharge you may see 70–80% of that number.
Forgetting the kVA–kW conversion. A 100 kVA UPS at 0.8 PF supports 80 kW, not 100 kW.
Not derating for battery age. Plan for 80% of new capacity toward end-of-life; batteries age faster than most MOPs assume.

Warn Runtime is a design target, not an SLA. Actual autonomy depends on temperature, discharge rate, cell balance, and age. Confirm against the battery vendor's discharge tables at the real planned load.

FAQ

What is UPS runtime?

How long a UPS can support its connected load from batteries after utility power is lost.

How long should a UPS run?

No universal target. Many sites design for 5–15 minutes to bridge generator startup; remote or unmanned sites may need longer.

What affects UPS runtime?

Battery voltage and Ah, DoD, UPS efficiency, PF, temperature, age, discharge rate, and actual load profile during the outage.

Why use 80% depth of discharge?

To avoid depending on the last 20% of capacity, which is least reliable due to aging and temperature effects.

How does load affect runtime non-linearly?

High loads reduce usable Ah (Peukert's law), so runtime drops faster than a perfectly linear estimate suggests.

Sources

IEEE 1184 — Selection and Sizing of Batteries for UPS IEEE 485 — Sizing Lead-Acid Batteries for Stationary Applications Mfr C&D Technologies / EnerSys discharge curve datasheets NEMA UPS product standards Uptime Ride-through & autonomy guidance

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