Diesel Generator Sizing Guide: How to Calculate DG Set KVA Requirement
Selecting the right DG set size is critical — too small and it trips on overload when large motors start; too large and it wastes capital and fuel. This guide shows the complete method for calculating diesel generator KVA, including motor starting load that most engineers miss.
A diesel generator must meet two separate requirements: steady-state running load (kVA at normal operation) and transient starting load (kVA when the largest motor is started across-the-line or by a star-delta starter). The starting kVA demand often determines the generator size — not the running load.
This guide covers the sizing methodology for standby (emergency) generators and prime power generators for industrial facilities.
Step 1: List All Loads on Generator
Not all loads in the facility may need to be connected to the generator. Categorise loads:
Critical loads (must run during power outage): • Safety systems: fire pumps, emergency lighting, fire alarm panel, PA system • Process continuity: critical process pumps, control systems, servers, UPS-fed equipment • Environmental compliance: effluent treatment plant pumps, air pollution control systems
Non-critical loads (can shed during outage): • HVAC (except critical areas), general lighting, non-essential utilities
For each critical load, note: • Rated kW (output or input — use input kW for equipment, output for motors) • Power factor • Starting method (DOL, star-delta, VFD, soft starter)
Step 2: Calculate Running kVA
Running kVA = Σ (kW / PF) for all critical loads running simultaneously
Apply a demand factor (not all critical loads will run at 100% simultaneously): • Process pumps: 0.85–0.95 (most running, some on standby) • Lighting: 1.0 (all on during outage) • HVAC: 0.70–0.80 • Welding and process equipment: 0.50–0.70
Running kVA (design) = Total connected kVA × Demand factor
The generator must deliver this kVA continuously at its rated power factor (typically 0.8 pf for diesel generators).
Step 3: Calculate Motor Starting kVA (Critical)
This is the step most engineers under-estimate. When a motor starts DOL (direct on-line), the starting current is 6–8× full-load current — a corresponding spike in kVA demand. The generator voltage dips under this load; if the dip exceeds 15–20%, other equipment trips and the generator itself may go into overload protection.
Starting kVA for largest motor: Starting kVA = Motor kW × Starting kVA factor
Starting kVA factors (per motor starting method): • DOL start: 6 × rated kVA of motor • Star-delta (Y-Δ): 2 × rated kVA • Soft starter: 1.5–2 × rated kVA • VFD: 1.0–1.5 × rated kVA (most controlled)
Design generator for: kVA (generator) ≥ Running kVA + Starting kVA of largest motor (The starting kVA replaces the running kVA of that motor during the start transient)
Step 4: Select Generator Rating
Standard diesel generator ratings (kVA at 0.8 pf): 7.5, 10, 15, 20, 25, 30, 40, 50, 62.5, 82.5, 100, 125, 160, 200, 250, 320, 400, 500, 625, 800, 1000, 1250, 1500, 2000 kVA
Select the rating above the higher of: (a) Running load: total running kVA / 0.75 (targeting 75% loading at normal operation) (b) Starting load: running kVA (excluding starting motor) + starting kVA of largest motor
Also specify: • Standby rating (for emergency backup: rated for 10% overload for 1 hr in 12 hr) • Prime power rating (for continuous use: rated at 100% for unlimited hours) • Standby rating is typically 10% higher than prime rating
Fuel consumption rule of thumb: 0.25–0.33 litres of diesel per kWh generated at 75–80% load.
Step 5: Size the Alternator and Control Panel
The diesel engine drives an alternator (synchronous generator). Key alternator specifications: • Rated kVA at 0.8 pf lagging • Voltage: 415 V (3-phase, 4-wire) for most industrial applications • Frequency: 50 Hz (India) • Winding: pitch factor (1/6 pitch or 2/3 pitch) for harmonic reduction when driving VFD loads • Protection class: IP21 for indoor, IP44 for outdoor or dusty environments • Voltage regulation: ±0.5% with AVR (automatic voltage regulator)
The AMF (Automatic Mains Failure) panel detects mains failure and automatically starts the generator and transfers load within 10–30 seconds. Specify ATS (Automatic Transfer Switch) with both electrical and mechanical interlocks to prevent paralleling with the grid.
Worked Example: Factory DG Set Sizing
Critical loads for a pharmaceutical factory: • 3× 22 kW process pumps (DOL start) = 66 kW, PF 0.85 → 77.6 kVA • 1× 55 kW chiller compressor (VFD start) = 55 kW, PF 0.90 → 61.1 kVA • Lighting and controls = 30 kW, PF 0.95 → 31.6 kVA • ETP plant: 15 kW, PF 0.85 → 17.6 kVA • IT/UPS loads: 20 kW, PF 0.95 → 21.1 kVA Total connected: 186 kW / 209 kVA. Demand factor 0.85 → running kVA = 178 kVA.
Largest DOL motor: 22 kW pump. Starting kVA = 22/0.85 × 6 = 155 kVA. Generator kVA ≥ (178 − 22/0.85) + 155 = (178 − 25.9) + 155 = 307 kVA. At 75% loading: 307 / 0.75 = 409 kVA → Select 400 kVA (check), or 500 kVA for margin.
Fuel cost: 400 kVA × 0.8 pf × 0.75 loading = 240 kW output × 0.30 L/kWh = 72 L/hr at ₹95/L = ₹6,840/hr.
Frequently Asked Questions
How do I calculate the KVA required for a diesel generator?
Step 1: List all critical loads (kW and PF) → sum running kVA = Σ(kW/PF). Step 2: Apply demand factor (0.8–0.9). Step 3: Calculate starting kVA for the largest motor (DOL: 6× motor kVA; star-delta: 2×; VFD: 1.2×). Step 4: Generator kVA ≥ (running kVA − motor running kVA) + motor starting kVA. Step 5: Select next standard rating above this value / 0.75 (for 75% loading target). Example: 200 kVA running load, 55 kW DOL motor (65 kVA) → (200 − 65) + 6×65 = 135 + 390 = 525 kVA → select 625 kVA.
What is the difference between standby and prime power rating for a DG set?
Standby rating (ESP — Emergency Standby Power): maximum power available for variable load when grid power fails. Typically rated for 10% overload for 1 hour in every 12 hours, with average load not exceeding 70% of standby rating. Not for continuous use. Prime power rating (PRP): maximum power available for continuous variable load with no grid connection. Rated for unlimited hours at 100% load; 10% overload for 1 hour in every 12 hours. Standby rating is typically 10% higher than prime rating for the same engine. Always specify which rating you need.
What fuel consumption should I expect from a diesel generator?
Specific fuel consumption (SFC) for modern diesel gensets: at 75% load: 0.26–0.30 L/kWh; at 100% load: 0.24–0.28 L/kWh; at 50% load: 0.30–0.36 L/kWh (less efficient at partial load). Rule of thumb: 0.25–0.33 L/kWh at 75–80% load. Example: 500 kVA genset at 75% load (375 kVA × 0.8 pf = 300 kW): fuel consumption = 300 × 0.28 = 84 L/hr. At ₹95/L diesel: ₹7,980/hr running cost.
Why does motor starting load determine generator size?
When a motor starts DOL (direct on-line), the starting current is 6–8× full-load current, creating a starting kVA of 6–8× rated motor kVA. This transient load causes the generator terminal voltage to dip. If the voltage dip exceeds 15%, other equipment connected to the generator (PLCs, contactors, lighting) may trip or malfunction. To limit voltage dip to acceptable levels, the generator alternator reactance (Xd″) and engine inertia must absorb the starting transient — this usually requires the generator to be 3–4× the starting motor's kVA for DOL starting. Use VFDs or soft starters to dramatically reduce starting kVA and allow smaller generators.