Battery Backup Runtime Calculator: Estimate UPS and Backup Power Duration

Runtime is primarily determined by total Ah capacity, discharge rate (C-rate), and inverter efficiency. At higher loads (expressed as a percentage of rated VA), runtime decreases non-linearly due to Peukert's effect. A load at 50% of rated VA typically provides 2-3x the runtime of a load at 100%, not the linear 2x prediction.

Peukert's law states that battery capacity decreases as the discharge rate increases: C = I^n * t, where n (1.1-1.3) is the Peukert exponent. A battery rated at 100Ah at C/20 rate delivers less than 100Ah at C/5 or C/1 rates. Most UPS runtime calculators use an effective capacity curve rather than a fixed Ah value to account for this non-linearity.

VA (apparent power) and W (real power) differ by the power factor of the load. For a UPS with PF=0.9, a 1000VA rated unit delivers 900W. Always size UPS by the higher of the two: use VA for the inverter rating and W for battery Ah calculations. Non-linear loads (servers, UPS inputs) can reduce effective PF to 0.6-0.7, significantly reducing effective capacity.

Battery capacity decreases approximately 1% per degree Celsius above 25°C. A battery bank at 35°C delivers only 90% of rated capacity. At 0°C it delivers approximately 70-80%. Cold temperatures increase runtime for a given load but reduce available capacity. Most UPS manufacturers rate capacity at 25°C. Always derate for elevated temperatures per IEEE1184 guidelines.

For UPS and backup power: Lead-acid (VRLA/AGM) provides 200-500 full cycles at 50% DoD. Lithium-ion (LiFePO4) provides 2000-5000 cycles at 80% DoD with 3-5x the energy density. For standby backup, lithium offers better TCO below 5 years due to longer life and deeper discharge capability. For infrequent power outages, lead-acid remains cost-effective below 2kVA.

Replace UPS batteries when: (1) the battery indicator shows fault/replace; (2) runtime at full load drops below 50% of original specification; (3) batteries are older than 3-5 years for VRLA or 8-10 years for lithium; (4) ambient temperature has consistently exceeded 30°C. IEEE1184 recommends annual battery capacity testing and replacement when capacity falls below 80% of rated.