UPS / Inverter Battery Backup Calculator
Estimate battery-bank Ah and inverter or UPS capacity for essential home and office loads.
Efficiency and sizing assumptions
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How battery Ah and inverter size are calculated
Battery energy (Wh) = load W × backup hours ÷ (conversion efficiency × allowed discharge × available-capacity factor). Required bank Ah = battery Wh ÷ bank nominal voltage. Percentage factors are converted to fractions. Battery sizing uses the actual continuous load; power headroom applies only to inverter sizing.
Minimum continuous output W = load × (1 + headroom). Minimum VA is the larger of design W ÷ load power factor and design W ÷ product W/VA ratio. Verify both ratings on the product label. The example VA class rounds up to one of 600, 800, 1000, 1500, 2000, 3000, 5000, 7500 or 10000 VA; it is not a product recommendation.
Home and office backup planning
Home and office presets are editable example totals, not measured appliance schedules. Add only essential loads and measure their operating watts. A higher VA rating alone does not guarantee more backup time. Desktop PCs, networking and servers also need suitable waveform and transfer-time performance. Hours of office backup may require an approved external-battery UPS.
This energy model uses 12 V lead-acid or 12.8 V LiFePO4 battery units. Series connections increase voltage; parallel strings increase Ah. The 12/24/48 V classes therefore use nominal lithium voltages of 12.8/25.6/51.2 V. Only use a battery and inverter combination explicitly supported by the manufacturers.
Planning estimate only. Batteries are assumed fully charged, matched and initially at the stated available-capacity factor. Defaults are editable planning assumptions, not guarantees. Lead-acid capacity falls at high discharge rates (Peukert effect), and actual runtime depends on temperature, cutoff voltage, ageing and discharge curves. Idle consumption, motor starting surge and nonlinear battery behaviour are not explicitly modelled. Confirm the manufacturer’s runtime tables, charger capacity, battery/BMS discharge limits, maximum permitted series/parallel configuration and DC protection before purchase or installation.
References: APC / Schneider UPS sizing guide and Victron usable battery capacity guide.
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Worked example
For the editable 350 W, four-hour example, using 85% efficiency, 50% allowed discharge and 80% available capacity, nominal energy = 350 × 4 ÷ (0.85 × 0.50 × 0.80) = 4,117.65 Wh. At 12 V this is 343.14 Ah, displayed rounded upward as 344 Ah. Three parallel 150 Ah units give an arithmetic 450 Ah bank and 5.25 hours in this simplified model. The parallel connection requires manufacturer approval and designed protection. A 25% inverter headroom gives 437.5 W and 546.875 VA at the stated 0.8 ratios; the example class rounds to 600 VA. These results are not a battery wiring plan or a runtime guarantee.