LoadCairn / UPS runtime

UPS battery runtime estimate

Adjust the electrical load and battery-bank inputs to see a transparent nominal-energy estimate. Inputs and results stay in this browser; tagged campaign visits and completed calculations are counted without visitor identifiers.

Inputs

Illustrative example · editable

Starts with 4 × 12 V, 100 Ah batteries in one series string, a 1,000 W load, 90% efficiency, and 80% estimated remaining capacity.

Electrical load

Use the expected real-power load, not UPS VA rating.

Connected loadW

Battery bank

Enter the rating for one battery, then describe the bank wiring.

Nominal voltage per batteryV

Rated capacity per batteryAh

Batteries in series per stringbatteries

Parallel stringsstrings

UPS and remaining capacity

Enter efficiency at the expected load and estimated capacity remaining after aging.

UPS efficiency%

Estimated remaining battery capacity%

Capacity remaining after aging; this is not battery age in years. The 80% starting value is illustrative, not universal.

Planning estimate

Estimated runtime

3 h 27 min

Rounded to the nearest minute. This is a nominal-energy estimate, not a guaranteed UPS runtime.

Estimate formula

Battery-bank energy = nominal volts per battery × rated Ah per battery × series batteries per string × parallel strings

12 V × 100 Ah × 4 batteries × 1 strings = 4,800 Wh

Battery-bank energy (Wh) × UPS efficiency fraction × remaining-capacity fraction ÷ load (W)

4,800 Wh × 90% × 80% ÷ 1,000 W = 3 h 27 min

Nominal bank energy: 4,800 Wh
Energy after factors: 3,456 Wh

Assumptions and limits

The bank starts fully charged.
The load is constant real power measured in watts.
Series and parallel capacity arithmetic is ideal.
Nameplate Ah is treated as available energy before the explicit efficiency and remaining-capacity factors.
No chemistry-specific Peukert or discharge-rate curve is applied.
No temperature correction is applied.
No inverter cutoff, battery-management limit, power-factor or inrush behavior, or manufacturer calibration is modeled.

Every result is an estimate for planning only—not a substitute for a licensed engineer’s design or manufacturer data.

Reference points

Check the estimate against equipment data

The simple energy model is a starting point. These manufacturer resources explain where measured or model-specific data should take over.

Schneider Electric / APC — UPS runtime estimate factors
Explains why age, temperature, load, and load characteristics affect actual runtime; APC notes VRLA capacity may reach approximately 80% near end of life.
Eaton — UPS runtime graphs
Use a published runtime curve for a specific UPS model for a more applicable estimate.
Trojan Battery — FAQ
Explains that rated amp-hours vary with discharge rate, so nameplate Ah divided by load is not a complete battery model.
Schneider Electric — UPS efficiency
Explains that efficiency varies with load; use the manufacturer’s battery-mode efficiency when available.

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