A correctly sized diesel generator (DG set) is the backbone of an uninterruptible power supply strategy for any commercial building — hotels, hospitals, offices, or data centres. Undersize it and you risk overload tripping during emergencies. Oversize it and you waste capital, increase fuel consumption, and cause premature engine wear from under-loading. This guide walks you through the complete generator sizing methodology used by professional MEP engineers.
Step 1 — Understand Load Categories
Not all loads need to run on generator power. Before sizing, categorise every electrical load in the building into three groups:
- Essential Loads (Critical): Must run within seconds of mains failure — life safety systems: fire pumps, emergency lighting, hospital critical care, server rooms, security systems. These are fed via the DG set through an Automatic Transfer Switch (ATS).
- Important Loads (Priority): Need power within 1–2 minutes — lifts, HVAC for occupied spaces, kitchen equipment, main lighting circuits. These are switched on progressively after the DG starts.
- Non-essential Loads: Can be shed during power outage — decorative lighting, non-critical equipment, car park ventilation. These remain off during generator operation.
Step 2 — Build a Load Schedule
The load schedule is the core document for generator sizing. List every load that will run on the generator, with its rated power, demand factor, and power factor:
| Load Description | Rated kW | Demand Factor | Actual kW | PF | kVA |
|---|---|---|---|---|---|
| Fire pump (22 kW motor) | 22 | 1.00 | 22.0 | 0.85 | 25.9 |
| Lifts × 2 (15 kW each) | 30 | 0.60 | 18.0 | 0.80 | 22.5 |
| HVAC AHU (30 kW) | 30 | 0.80 | 24.0 | 0.85 | 28.2 |
| Lighting (essential) | 15 | 0.90 | 13.5 | 1.00 | 13.5 |
| IT / Server room | 20 | 0.90 | 18.0 | 0.90 | 20.0 |
| Kitchen equipment | 10 | 0.70 | 7.0 | 0.95 | 7.4 |
| Total | 127 | — | 102.5 | — | 117.5 |
Step 3 — Apply Diversity Factor
In real buildings, not all loads reach their peak demand simultaneously. The diversity factor accounts for this. For commercial buildings, a diversity factor of 0.75–0.90 is typically applied to the sum of maximum demands:
Step 4 — Apply Safety Margin and Select Standard Size
Step 5 — Check Step-Load Capacity
After a mains failure, loads are added progressively (sequenced by ATS and load controllers). The critical check is: can the generator handle the largest single step-load addition without excessive voltage or frequency dip?
Use our Generator Sizing Calculator to build your load schedule digitally and instantly compute required kVA with diversity factors and safety margins applied.
Key Formulas for Generator Sizing
Power Factor Considerations
Most DG sets are rated at 0.8 power factor. If your connected load has a higher average PF (well-corrected capacitor banks), the generator can deliver more kW for the same kVA rating. However, if your loads are predominantly motors and fluorescent lighting without PF correction, the system PF may drop to 0.7 or lower — meaning the generator kW output is significantly reduced even at rated kVA.
Always calculate the weighted average PF of all loads on the generator and size accordingly. Consider adding automatic power factor correction (APFC) panels even on the DG side for large installations.
Altitude and Temperature Derating
Diesel generators lose rated output at high altitudes and high ambient temperatures because of reduced air density and reduced combustion efficiency:
| Condition | Derating Factor |
|---|---|
| Sea level, 25°C (base) | 1.00 (no derating) |
| Every 500m altitude above 1000m | Derate by ~3% |
| Every 5°C above 40°C ambient | Derate by ~1.5% |
| Example: 1500m altitude, 45°C | ~6% altitude + 1.5% temp = 7.5% derate |
Common Generator Sizing Mistakes
- Summing nameplate ratings without demand factors: A motor rated 22 kW rarely runs at full nameplate load. Always apply demand factors from actual operating data or standard references.
- Ignoring load sequencing: Automatic load sequencing (adding loads in stages after generator startup) significantly reduces step-load stress and allows a smaller DG set to handle the total load.
- Not accounting for harmonic loads: VFDs, UPS systems, and LED drivers generate harmonics that increase generator loading beyond the nameplate kW. Apply a harmonic derating factor for installations with significant non-linear loads.
- Undersizing for future growth: Always include at least 20% spare capacity for future load additions. It is far cheaper to upsize a generator during initial installation than to replace it later.
Conclusion
Generator sizing is a systematic process: categorize loads, build a detailed load schedule, apply diversity and demand factors, check step-load capability, and select the next standard size with a growth margin. The goal is a generator that runs at 70–80% of rated load under normal backup conditions — efficient, reliable, and ready for emergencies.
Use the MEPMate Generator Sizing Calculator to speed up your load schedule and get instant kVA recommendations for your next project.