UPS runtime formula #

For an idealized energy-balance screen:

Runtime (minutes) ≈ Vstring × Ahstring × parallel strings × efficiency × usable fraction ÷ load W × 60

Where:

  • Vstring is the nominal DC voltage of one complete series string;
  • Ahstring is the amp-hour rating of that series string;
  • parallel strings is the number of identical strings connected in parallel;
  • efficiency is the assumed DC-to-AC conversion efficiency as a decimal;
  • usable fraction accounts for the portion of nominal energy used in the screen;
  • load W is measured or estimated real power supported during the outage.

Series-connected batteries increase string voltage but do not add amp-hour capacity. Parallel identical strings add amp-hour capacity. This distinction prevents the common error of counting the batteries within one series string as multiple parallel strings.

Estimate 5 kW runtime with one 48 V, 100 Ah string →

This guide owns the formula, assumptions and limitations. The UPS Runtime Calculator owns instant estimates.

Best for: screening runtime from a known battery-string configuration and real load. Not suitable for: approving a battery cabinet, fire-code design or guaranteed runtime without the exact UPS manufacturer's data. Browse related workflows on the UPS Calculator Hub.

Worked example: one 48 V, 100 Ah string #

Assume four 12 V, 100 Ah batteries are connected in series:

  • string voltage = 4 × 12 V = 48 V
  • string capacity = 100 Ah, not 400 Ah
  • parallel strings = 1
  • screen efficiency = 0.90
  • usable fraction = 1.00 for this ideal comparison
  • AC load = 5,000 W

Nominal DC energy:

48 V × 100 Ah × 1 = 4,800 Wh

Idealized AC-side energy:

4,800 Wh × 0.90 = 4,320 Wh

Runtime screen:

4,320 Wh ÷ 5,000 W × 60 = 51.8 minutes

The result is an energy screen, not an expected acceptance-test value. Actual runtime can be lower because battery voltage falls under load and available energy depends on discharge rate, cutoff voltage, temperature, condition and inverter behavior.

What changes the result #

Actual load profile #

Use real power in watts or kW during battery operation. Nameplate totals can overstate or understate the simultaneous load. If equipment shuts down in stages, model each interval or use the worst required sequence.

Battery discharge rate #

Ah is normally specified at a stated discharge duration and cutoff voltage. A battery discharged at UPS power rates may not deliver nominal Ah according to a linear Wh calculation. Use the battery manufacturer's constant-power data or the UPS model's published runtime curve.

Temperature, condition and end-of-life criteria #

Do not apply a generic age-by-year percentage. Use the battery or UPS manufacturer's temperature factors, monitoring data, test results and the project's end-of-life requirement.

UPS efficiency and cutoff #

Efficiency changes with load and operating mode, and the UPS stops at a defined DC cutoff. Use the exact model's data rather than a generic efficiency when making a procurement or compliance decision.

One string versus four strings #

With the same 48 V, 100 Ah battery string and 5 kW load:

  • one parallel string: nominal energy = 4.8 kWh; ideal screen ≈ 51.8 min at 90% efficiency;
  • four parallel strings: nominal energy = 19.2 kWh; ideal screen ≈ 207.4 min at 90% efficiency.

Four individual 12 V batteries connected in series make one 48 V string. Four parallel 48 V strings would require sixteen of those 12 V batteries in this simplified example.

Formula versus OEM runtime curve #

Question Energy formula OEM data
Quick comparison from known V/Ah/load Useful Useful if exact model is known
High-rate discharge and cutoff behavior Simplified Model-specific
Battery cabinet/string compatibility Not established Required
Final expected runtime Screening only Use published curve and project verification

When the design starts with a required number of minutes and asks for battery Ah, use the UPS Battery Calculator and UPS battery sizing guide. When choosing the UPS kW/kVA frame, use the industrial UPS sizing guide.

Common mistakes #

  1. Adding Ah for batteries in series: series adds voltage; parallel strings add Ah.
  2. Using kVA as load watts: use real output load in W or kW for the energy calculation.
  3. Assuming runtime scales perfectly with load: high-rate battery behavior and inverter losses are nonlinear.
  4. Using nominal battery data as an end-of-life guarantee: apply the selected manufacturer's criteria.
  5. Skipping the exact UPS curve: the simple formula cannot model the full DC system.

Technical source #

  • Eaton UPS sizing and runtime guidance — load, runtime and model-selection context. Final verification should use the selected UPS model's runtime charts and approved battery configuration.

FAQ #

How do I calculate UPS runtime from battery Ah?

Multiply string voltage by string Ah and the number of parallel strings, apply documented efficiency and usable-energy assumptions, divide by real load watts, then multiply hours by 60. Verify against the OEM runtime curve.

Do batteries in series add their Ah ratings?

No. Series batteries add voltage while the string Ah stays equal to one battery's Ah rating, assuming identical batteries. Identical parallel strings add Ah.

Why does the manufacturer runtime differ from the formula?

The manufacturer curve includes model-specific cutoff, inverter behavior and battery discharge performance that a linear Wh calculation simplifies.

Next step #

Enter the exact string voltage, Ah, parallel-string count and measured load in the UPS Runtime Calculator, then confirm the result with the selected model's battery and runtime data.