Quick answer: UPS VA = connected load (W) ÷ power factor, with a 20–25% margin. Battery capacity for backup: Ah = (Load W × backup hours) / (V_batt × efficiency × DoD).
Two steps: VA and battery
UPS VA = Load (W) / PF × (1 + margin)
Battery Ah = (Load W × t_hours) / (V_dc × η × DoD)
η ~ 0.9 inverter efficiency, DoD ~ 0.8Worked example
Load 3000 W, PF 0.8: VA = 3750, +20% → select a 5 kVA UPS. For 30 min backup at 48 V DC: Ah = (3000×0.5)/(48×0.9×0.8) ≈ 43 Ah battery bank.
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How do you calculate UPS VA rating?
Divide the connected load in watts by the power factor to get VA, then add a 20-25% margin for growth and inrush. Round up to the next standard UPS rating.
How is UPS battery backup time calculated?
Battery amp-hours equal the load in watts times backup hours, divided by the DC bus voltage, inverter efficiency and allowable depth of discharge. More Ah or higher DC voltage gives longer backup.