🔌 ELECTRICAL

UPS Sizing Calculator (kVA + Battery Capacity)

Size UPS / online inverter — kVA rating and battery Ah for required backup time. IS 16242 / IEC 62040 compliant. Free tool.

📐 Standard: IS 16242 / IEC 62040
✅ Free to use
📄 PDF export
📱 Mobile friendly

ℹ️ About This Calculator

A UPS (Uninterruptible Power Supply) provides clean, conditioned power and bridges power outages. In India, where grid power outages can last minutes to hours, correct UPS sizing — both kVA capacity and battery Ah — is critical. An undersized UPS overloads and shuts down on transfer; an undersized battery bank exhausts before the outage ends.

IS 16242 covers UPS systems for general use. IEC 62040-3 classifies UPS types: Class 1 (Online/double conversion — best protection), Class 2 (Line-interactive), Class 3 (Standby/offline — basic). For servers, medical equipment, and critical industrial processes: always use Class 1 online double-conversion UPS. For computers and office equipment: Class 2 line-interactive is acceptable. VRLA (sealed lead-acid) batteries have 3–5 year design life; Li-ion has 8–10 year life at higher initial cost.

📐 UPS Sizing Formula (IS 16242)

IS 16242 / IEC 62040

UPS kVA Rating:
  kVA = (Total VA load) × Derating factor (1.2–1.25)

Battery Ah Calculation:
  I_discharge = (kW_load / η_inverter) / V_battery_bank
  Ah = I_discharge × t_backup / DoD

Where:
  η_inverter = inverter efficiency (0.90–0.95)
  V_battery  = battery bank voltage (48V, 96V, 192V...)
  t_backup   = required backup time (hours)
  DoD        = depth of discharge (0.8 for VRLA, 0.9 for Li-ion)

Battery Bank Configuration:
  Cells in series = V_bank / V_cell (12V blocks or 2V cells)
  Strings in parallel = Ah_required / Cell_Ah_rating

❓ Frequently Asked Questions

How do I calculate UPS size for my office? +
Add up the VA rating of all equipment to be protected: computers (400–600 VA each), servers (1000–3000 VA), monitors (100–200 VA), networking equipment (200–500 VA). Multiply by 1.25 for derating. For 10 computers at 500VA + 1 server at 2000VA + networking at 500VA: Total = 5000+2000+500 = 7500 VA × 1.25 = 9375 VA → select 10 kVA UPS.
What battery backup time should I design for? +
For servers and data centres: 10–15 minutes (enough to shut down gracefully or switch to generator). For hospitals: 30–60 minutes minimum, 4 hours for critical areas. For telecom: 8–24 hours. For office computers: 15–30 minutes (enough to save work and shut down). Indian data centres typically size for 20–30 minutes with generator backup.
How many years do UPS batteries last? +
VRLA (sealed lead-acid) batteries in UPS applications typically last 3–5 years in Indian conditions (due to temperature — every 10°C above 25°C halves battery life). Gel batteries last 5–7 years. Li-ion batteries last 8–10 years. Install UPS in air-conditioned spaces to maximize battery life. Replace batteries before end of life — a failed battery bank provides zero backup.
What is the difference between online and offline UPS? +
Online (double-conversion) UPS: converts incoming AC to DC, then back to AC continuously. Load always runs from inverter — zero transfer time, perfect power conditioning. Offline (standby) UPS: load runs directly from mains; switches to inverter only on failure — 8–20ms transfer time, basic protection only. Online UPS is essential for servers and medical equipment; offline is acceptable for basic PC protection.
Can I connect a generator to a UPS? +
Yes, but requires configuration. Most online UPS systems accept generator input, but generator frequency and voltage variation during startup may trigger UPS transfer to battery before the generator stabilizes (10–30 seconds). Configure the UPS with extended bypass window (widen the input voltage and frequency acceptance range) to work seamlessly with generators. Confirm compatibility with your UPS manufacturer.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →

🔌 UPS Sizing Calculator
Reference: IS 16242 / IEC 62040