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LoadCairn / UPS runtime

UPS runtime calculator.

Adjust the electrical load and battery-bank inputs to see a transparent nominal-energy estimate. Inputs and results stay in this browser. We count page views and completed calculations for basic analytics; see the privacy policy.

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.

Battery bank

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

UPS and remaining capacity

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

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
Checking Pro access…Download UPS runtime reportExport CSV

Estimates for planning only, not a substitute for a licensed engineer’s design or manufacturer data.

Calculation details

Estimate formula

  1. Step 1: Battery-bank energy = nominal volts per battery × rated Ah per battery × series batteries per string × parallel strings
  2. Step 2: Battery-bank energy (Wh) × UPS efficiency fraction × remaining-capacity fraction ÷ load (W)
  3. Step 3: Usable energy (Wh) = battery-bank energy × (UPS efficiency % ÷ 100) × (remaining battery capacity % ÷ 100)
  4. Step 4: Runtime (hours) = usable energy (Wh) ÷ connected load (W); displayed runtime is rounded to the nearest minute

Current calculation

  1. 12 V × 100 Ah × 4 batteries × 1 strings = 4,800 Wh
  2. 4,800 Wh × (90% ÷ 100) × (80% ÷ 100) = 3,456 Wh
  3. 3,456 Wh ÷ 1,000 W = 3.46 h; rounded to the nearest minute = 3 h 27 min
Nominal bank energy
4,800 Wh
Usable energy after factors
3,456 Wh
Runtime in seconds
12,441.6 seconds

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.

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.

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

Non-guaranteed planning estimate. No chemistry-specific Peukert or discharge-rate curve, temperature correction, inverter cutoff or battery-management limits, power factor or inrush, or manufacturer/model calibration is included. For equipment decisions, use model-specific manufacturer data.

Need a target runtime instead? UPS battery sizing →

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LoadCairn / UPS runtime

UPS runtime estimate

Current estimate from the values entered in the calculator.

Estimated runtime

3 h 27 minRounded to the nearest minute

Current inputs

Connected load
1,000 W
Nominal voltage per battery
12 V
Rated capacity per battery
100 Ah
Batteries in series per string
4 batteries
Parallel strings
1 string
UPS efficiency
90%
Estimated remaining battery capacity
80%

Formula and calculation

  1. Battery-bank energy = nominal volts per battery × rated Ah per battery × batteries in series × parallel strings
  2. Usable energy = battery-bank energy × (UPS efficiency % ÷ 100) × (remaining capacity % ÷ 100)
  3. Runtime (hours) = usable energy (Wh) ÷ connected load (W); displayed runtime is rounded to the nearest minute
  1. 12 V × 100 Ah × 4 batteries × 1 string = 4,800 Wh
  2. 4,800 Wh × (90% ÷ 100) × (80% ÷ 100) = 3,456 Wh
  3. 3,456 Wh ÷ 1,000 W = 3.456 hours = 3 h 27 min

Assumptions

  • 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.

Planning limitations

  • This is a nominal-energy estimate; it does not apply a chemistry-specific Peukert effect, discharge-rate curve, or manufacturer runtime curve.
  • It does not model battery temperature or aging beyond the estimated remaining-capacity percentage entered above.
  • It does not model inverter cutoff, battery-management limits, power factor, inrush, transient load, or changing load over time.
  • It does not validate equipment compatibility, installation conditions, safety, electrical-code compliance, or a manufacturer warranty.
  • Use model-specific manufacturer runtime data and qualified electrical review for equipment selection or design decisions.

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

BICSI DCDC Study Guide and Practice Exam

Method / Battery bank

Read the battery-bank inputs

Battery voltage multiplied by amp-hour (Ah) capacity gives nominal energy in watt-hours (Wh). Ah alone is not an energy value, so use the battery voltage with its rating. For matching batteries, series connections add voltage while the Ah capacity of each string stays the same; parallel strings add capacity. For example, four 12 V, 100 Ah batteries in series make one 48 V, 100 Ah string. A second matching string in parallel adds capacity. Keep the series-battery count separate from the number of parallel strings.

UPS efficiency and remaining battery capacity both reduce the nominal energy available to the load. Lower values reduce the estimate; use 100% for either factor only when there is a defensible basis for assuming no conversion loss or no capacity reduction.

Enter real power in watts (W), not apparent power in volt-amperes (VA). Use a measured or documented load and label whether it represents normal operation, a peak, or a modeled future condition so the scenario remains clear.

Convert each entered percentage to a fraction by dividing it by 100 before multiplying. In the displayed sample, 207.36 minutes rounds to 207 minutes, or 3 h 27 min.

Application / Ride-through

Compare the estimate with equipment data

Compare the required ride-through time with the selected UPS and battery manufacturer’s published runtime at the expected load. Reassess the comparison when the real-power load grows or available battery capacity falls. Before changing equipment, confirm UPS and battery compatibility, charging limits, and manufacturer guidance for the intended configuration.

Reference / Example configurations

Typical UPS runtime scenarios

These planning-only estimates use the calculator formula and the assumptions shown in every row. Real runtime varies with battery age and inverter efficiency. Battery discharge curves, temperature, and UPS operating conditions can also affect results; check the UPS and battery manufacturer’s data for equipment-specific runtime.

Nominal Wh = battery V × battery Ah × batteries in series × parallel strings. Usable Wh = nominal Wh × (efficiency ÷ 100) × (remaining capacity ÷ 100). Runtime hours = usable Wh ÷ load W.

Scroll the table horizontally to read each calculation.

Planning estimates for UPS runtime using different battery banks and loads
Setup and bankLoad and factorsNominal energyUsable energyEstimated runtime
Small network closet24 V, 200 Ah bank2S × 2P of 12 V, 100 Ah batteries500 W load90% efficiency · 80% remaining12 V × 100 Ah × 2 × 2 = 4,800 Wh4,800 Wh × (90 ÷ 100) × (80 ÷ 100) = 3,456 Wh3,456 Wh ÷ 500 W = 6.912 h6 h 55 min
Single rack, light load48 V, 100 Ah bank4S × 1P of 12 V, 100 Ah batteries300 W load90% efficiency · 80% remaining12 V × 100 Ah × 4 × 1 = 4,800 Wh4,800 Wh × (90 ÷ 100) × (80 ÷ 100) = 3,456 Wh3,456 Wh ÷ 300 W = 11.52 h11 h 31 min
Single rack, standard load48 V, 200 Ah bank4S × 2P of 12 V, 100 Ah batteries800 W load90% efficiency · 80% remaining12 V × 100 Ah × 4 × 2 = 9,600 Wh9,600 Wh × (90 ÷ 100) × (80 ÷ 100) = 6,912 Wh6,912 Wh ÷ 800 W = 8.64 h8 h 38 min
High-density rack48 V, 300 Ah bank4S × 2P of 12 V, 150 Ah batteries1,500 W load92% efficiency · 75% remaining12 V × 150 Ah × 4 × 2 = 14,400 Wh14,400 Wh × (92 ÷ 100) × (75 ÷ 100) = 9,936 Wh9,936 Wh ÷ 1,500 W = 6.624 h6 h 37 min
Small equipment-room UPS96 V, 200 Ah bank8S × 2P of 12 V, 100 Ah batteries2,500 W load90% efficiency · 80% remaining12 V × 100 Ah × 8 × 2 = 19,200 Wh19,200 Wh × (90 ÷ 100) × (80 ÷ 100) = 13,824 Wh13,824 Wh ÷ 2,500 W = 5.5296 h5 h 32 min
Larger facility UPS192 V, 200 Ah bank16S × 2P of 12 V, 100 Ah batteries4,000 W load92% efficiency · 80% remaining12 V × 100 Ah × 16 × 2 = 38,400 Wh38,400 Wh × (92 ÷ 100) × (80 ÷ 100) = 28,262.4 Wh28,262.4 Wh ÷ 4,000 W = 7.0656 h7 h 4 min

Worked example / 48 V bank

Calculate one runtime estimate

Planning estimate: a measured 800 W load is supplied by four 12 V, 100 Ah batteries in series and one parallel string. Assume 90% UPS efficiency and 80% remaining battery capacity.

  1. Nominal bank energy: 12 V × 100 Ah × 4 series batteries × 1 parallel string = 4,800 Wh. The bank is 48 V, 100 Ah.
  2. Usable energy: 4,800 Wh × (90 ÷ 100) × (80 ÷ 100) = 3,456 Wh.
  3. Runtime in hours: 3,456 Wh ÷ 800 W = 4.32 hours.
  4. Runtime in minutes: 4.32 h × 60 min/h = 259.2 minutes, which rounds to 259 minutes using the calculator’s nearest-minute display.

Estimate: 259.2 minutes (4.32 hours) before display rounding. This is for planning only, not a substitute for a licensed engineer’s design or manufacturer data.

Questions / Runtime planning

UPS runtime questions

How long will a UPS run a server rack?

It depends on the rack’s real power draw and the compatible battery bank’s voltage, amp-hour capacity, series and parallel arrangement, UPS efficiency, and remaining battery capacity. Enter measured watts and the documented battery configuration in the calculator, then compare the estimate with the UPS and battery manufacturer’s runtime data for that load.

How do I size a UPS for 30 minutes of runtime?

Start with the measured load in watts. For 30 minutes, the load needs load watts × 0.5 hours of usable energy. To estimate nominal battery energy, divide that amount by the efficiency fraction and remaining-capacity fraction you plan to use, then select a battery bank compatible with the UPS. Check the result against manufacturer data and have the design reviewed by a licensed engineer.

Why does runtime drop as batteries age?

A battery that has lost capacity stores less usable energy than when it was new, so it can support the same load for less time. The remaining-capacity input represents that reduction as an estimate; use battery test results and manufacturer guidance for an equipment-specific value.

Does inverter efficiency matter?

Yes. Some battery energy is lost when the UPS converts DC battery power to AC power. Lower efficiency means less energy reaches the connected load and the estimated runtime is shorter. Use a supported efficiency value for the UPS and operating load when one is available.

What is the difference between VA and watts for runtime?

Watts measure real power used by the load; volt-amperes (VA) measure apparent power. This runtime calculation uses watts in its denominator, so use the load’s measured or documented watt value. The UPS must also stay within both its watt and VA ratings; use equipment data for the load power factor rather than guessing it.

How many batteries do I need to double runtime?

With the same load and compatible batteries, doubling parallel strings of the same voltage and capacity doubles nominal energy in this simplified model. Adding batteries in series changes bank voltage and is only appropriate when the UPS supports that voltage. Battery discharge behavior can make actual runtime differ, so confirm the arrangement and runtime with manufacturer data.

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