UPS Sizing Based on Electrical Load Calculator – IEC

Correct UPS sizing is essential to maintain electrical system reliability in industrial, commercial, and IT environments.
IEC standards offer a strong framework for load analysis, guiding engineers in making informed UPS decisions.

Extensive Table of Common UPS Sizing Values (IEC-Based)

Note: Always apply a 1.2-1.3 safety factor depending on environmental and operational conditions.

UPS Sizing Formulas (IEC-Compliant)

1. Apparent Power (S)

S (kVA) = P (kW) / PF

• P (kW): Active Power demand of the load
• PF: Power Factor (commonly between 0.7 and 0.95)

2. UPS Size with Safety Margin

UPS Size (kVA) = (Total Load in kW / PF) × Safety Factor

• Safety Factor: Typically 1.2 to 1.3

3. Battery Capacity Calculation

Battery Capacity (Ah) = (UPS Load (W) × Backup Time (hr)) / (Battery Voltage × Efficiency)

• UPS Load: Total load in Watts
• Backup Time: Desired autonomy (in hours)
• Battery Voltage: e.g., 48V, 96V, 192V
• Efficiency: UPS system efficiency (usually ~0.85 – 0.95)

4. Number of Batteries

Number of Batteries = Battery Voltage System / Voltage per Battery (typically 12V)

5. Energy Storage Requirement

Energy Required (Wh) = Load (W) × Backup Time (hr)

Explanation of Key Variables and Common Values

Real-World Application Example 1: Small Business Server Room

Scenario:

• Load = 10 kW
• Power Factor = 0.85
• Required Backup Time = 30 minutes
• Battery Bank Voltage = 192V
• Efficiency = 0.9
• Safety Factor = 1.25

Step-by-step:

1. UPS Size (kVA):

UPS Size = (10 / 0.85) × 1.25 = 14.7 kVA → rounded up to 15 kVA

2. UPS Load in W: 10,000 W
3. Energy Required (Wh):

10,000 × 0.5 hr = 5000 Wh

4. Battery Capacity (Ah):

= (10,000 × 0.5) / (192 × 0.9) = 5000 / 172.8 ≈ 28.9 Ah

Choose 2 × 12V, 100Ah batteries in series (to form 192V) ✓

Real-World Application Example 2: Hospital Diagnostic Equipment

Scenario:

• Load = 6 kW
• Power Factor = 0.95
• Required Backup Time = 1 hour
• Battery Voltage = 96V
• Efficiency = 0.92
• Safety Factor = 1.2

Step-by-step:

1. UPS Size:

= (6 / 0.95) × 1.2 = 7.57 → 8 kVA

2. Energy Required:

6000 × 1 = 6000 Wh

3. Battery Capacity:

6000 / (96 × 0.92) = 6000 / 88.32 ≈ 67.9 Ah

Use 96V bank (8 × 12V batteries in series), each 100Ah for margin ✓

Additional Example: Data Center Modular UPS

Scenario:

• Load = 20 kW
• PF = 0.9
• Backup Time = 10 minutes
• Voltage = 384V
• Efficiency = 0.93
• Safety Factor = 1.25

Calculations:

1. UPS Size:

(20 / 0.9) × 1.25 = 27.78 → 30 kVA

2. Energy Required:

20,000 × (10 / 60) = 3,333 Wh

3. Battery Capacity:

3,333 / (384 × 0.93) ≈ 9.35 Ah → Use 100Ah batteries × 32 (384V)

Best Practices and Additional Considerations

• IEC 62040: Ensure UPS systems comply with IEC 62040 standards.
• Derating: Apply derating for ambient temperature above 25°C.
• Redundancy (N+1/N+2): For critical loads, always consider redundancy.
• Modular UPS: Offers scalability for growing power needs.
• Harmonic Filtering: Important in systems with non-linear loads.
• Maintenance Bypass Switches: Allow UPS maintenance without downtime.
• Monitoring Systems: Integrate SNMP or Modbus monitoring for status alerts.
• Lifecycle Planning: Account for battery replacement (3-5 years typical).
• UPS Topologies: Choose between offline, line-interactive, and double-conversion based on criticality.
• Cooling Considerations: UPS rooms should be temperature controlled to avoid premature aging.
• Load Diversity: Consider the nature of varying loads across time (start-up surges, intermittent loads).
• Battery Type: Evaluate VRLA, Li-Ion, or NiCd based on budget, reliability, and lifecycle.

Authoritative Resources

• IEC 62040 UPS Standards
• IEEE Power Quality Standards
• APC UPS Sizing Tool
• Eaton UPS Sizing Guide
• Schneider Electric UPS Planning Whitepapers