🔋 ELECTRICAL

Generator Sizing Calculator (DG Set kVA)

Calculate generator kVA from connected loads with demand and power factors. Size DG sets for buildings and industrial applications. IS 10000.

📐 Standard: IS 10000 / IEEE
✅ Free to use
📄 PDF export
📱 Mobile friendly

ℹ️ About This Calculator

A diesel generator (DG set) must be sized to handle both the running load and the starting surge of connected equipment — particularly motors, which draw 5–7 times their rated current at start-up. Undersizing causes voltage dip and frequency drop at start; oversizing wastes fuel and causes "wet stacking" in diesel engines running at low load.

IS 10000 specifies methods of tests for internal combustion engines used in generator sets. For building backup power, a demand factor of 0.7–0.85 is typically applied since not all loads operate simultaneously. Critical facilities (hospitals, data centres) use a higher demand factor of 0.9–1.0. CPCB (Central Pollution Control Board) regulations in India mandate emission norms for DG sets — ensure the selected set meets CPCB norms applicable to your state.

📐 Generator Sizing Method (IS 10000)

IS 10000 / IEEE

Step 1 — Total connected kVA:
  kVA_total = Σ(kW_load / PF_load)

Step 2 — Apply demand factor:
  kVA_demand = kVA_total × Demand Factor (typically 0.7–0.85)

Step 3 — Motor starting kVA check:
  kVA_start = largest motor kW × starting kVA/kW (6–8× for DOL)
  Generator must handle: kVA_running + kVA_starting_largest_motor

Step 4 — Derate for altitude and temperature:
  Derate 3% per 300m above 1000m elevation
  Derate 1% per 5°C above 25°C ambient

Step 5 — Add 15–20% spare capacity for future loads

❓ Frequently Asked Questions

How do I calculate the kVA of a generator needed for my building? +
Sum all connected loads in kW, divide each by its power factor to get kVA, apply a demand factor (0.75–0.85 for commercial buildings), then add the starting kVA of the largest motor. Finally, add 20% for future load growth. Our calculator does all these steps automatically.
What is the difference between kVA and kW for a generator? +
kW is the actual working power (real power). kVA is the apparent power — it includes both working power and reactive power. kVA = kW / Power Factor. Generators are rated in kVA because they must supply both real and reactive current regardless of the load's power factor. A 100 kVA generator at 0.8 PF delivers 80 kW.
How much does a DG set derate at high altitude in India? +
Above 1000m elevation, diesel generators derate approximately 3% per 300m due to reduced air density (less oxygen for combustion). Locations like Bangalore (920m), Pune (560m), and Shimla (2200m) require derating consideration. For Shimla: roughly (2200-1000)/300 × 3% ≈ 12% derating.
What is wet stacking and how do I prevent it? +
Wet stacking occurs when a diesel generator operates at less than 30% of rated load for extended periods. Unburnt fuel and carbon accumulate in the exhaust. Prevent it by loading the generator to at least 50–75% of rated capacity. If low loads are unavoidable, run a periodic load bank test (1–2 hours at 75% load monthly).
Do I need a separate DG set for lifts and fire pumps? +
NBC 2016 and local fire safety regulations require that life safety systems (fire pumps, emergency lighting, lifts used for evacuation) be on a dedicated power source. While they can share a DG set with essential services, fire pump starters must be segregated so that a fault in another system cannot interrupt fire pump supply. Many states now require a separate dedicated generator for fire safety systems.

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

🔋 Generator Sizing Calculator
Reference: IS 10000 / IEEE