Sizing & runtime

UPS Battery Backup Runtime Calculator

Estimate how long a UPS battery string can carry a load, including inverter efficiency, the Peukert rate effect and an optional ageing factor.

Results are mathematical estimates from the values you enter — not measurements and not data about a specific product or vehicle.

Load on the UPS output

Real power in watts (not VA). Read it on the UPS display or with a power meter.

Batteries in series

Nominal voltage per battery

Rated capacity per battery

Strings in parallel

Rating hours of that capacity

20 for a C20 rating (most VRLA datasheets), 10 for C10.

Inverter efficiency

From the UPS datasheet (battery mode).

Peukert exponent

1.00 ignores the rate effect. For lead-acid take it from the datasheet or derive it from two published discharge times.

Remaining capacity (ageing)

Enter 80 to size for an end-of-life battery (the 1.25 ageing factor of IEEE 485 sizing practice).

How does the UPS Battery Backup Runtime Calculator work?

UPS runtime is the battery energy available at the discharge rate the load imposes, divided by the power drawn from the batteries. The calculator converts load watts into battery current (P ÷ (V × η)), applies Peukert's law to the rated capacity and multiplies by the usable and ageing fractions.

Formula

V(bank) = N(series) × V(battery)
I(string) = P ÷ (V(bank) × η) ÷ N(parallel)
t = H × (C ÷ (I(string) × H))^k × f(age)

Worked example

300 W, 2 × 12 V 9 Ah (C20), 90%, k = 1.10, 100% → V = 24 V, I = 13.89 A, t ≈ 0.46 h (≈ 28 min).

Limitations and warnings

UPS discharges are fast (minutes). Peukert's law extrapolated from a 20-hour rating is only an approximation at those rates; the battery maker's constant-power table (W per cell at 5–30 min) is the authoritative source.
Temperature, battery age and the UPS low-voltage cut-off change the real runtime.

Four values that are often confused

Nominal capacity (Ah): Charge a battery delivers under rated conditions, usually over 20 hours (C20) at 25 °C.
Usable capacity: The part of the nominal capacity you can use without damaging the battery — about 50% for lead-acid, 80–90% for LiFePO4.
Cranking current (CCA): Current deliverable for a short time in the cold to start an engine. It measures power, not stored charge, and cannot be converted to Ah.
Stored energy (Wh): Ah × voltage. The right unit to compare batteries of different voltages.