Getting Generator Sizing Wrong Is Expensive
We’ve seen the consequences of improper generator sizing more times than we can count. Last year, a mining company in Zambia ordered a 1000 kVA generator for their new processing facility. After installation, they discovered the unit was severely oversized — they were only using 350-400 kW on average. The result? The generator was running at 30-40% load, suffering from wet stacking (unburned fuel accumulating in the exhaust), consuming far more fuel per kWh than a properly sized unit, and requiring frequent maintenance.
On the flip side, we’ve worked with buyers who undersized their generators. A hotel in East Africa bought a 200 kVA unit that seemed adequate on paper, but when all kitchen equipment, air conditioning, and water pumps started simultaneously during peak hours, the generator kept tripping on overload. They ended up needing a second unit within months.
At Higenset, proper sizing is the first thing we discuss with every client. This guide walks you through the methodology we use — it’s not complicated, but it requires careful attention to detail.
Step 1: List All Your Equipment
The first step is straightforward: make a complete list of everything that will be connected to the generator. Don’t forget anything — even small loads add up.
Organize your list into three categories:
- Resistive loads: Lighting, electric heaters, computers, servers, TVs. These are the simplest — their running power equals their starting power.
- Motors and inductive loads: Air conditioning compressors, water pumps, fans, refrigerators, elevators. These draw significantly more power during starting than while running.
- Nonlinear loads: Variable frequency drives (VFDs), UPS systems, LED lighting drivers, computers with switching power supplies. These can cause harmonic distortion that affects generator performance.
For each item, record the rated power in kW or HP (1 HP ≈ 0.746 kW), the voltage, and whether it runs continuously or intermittently.
Step 2: Calculate Running Power
Add up the running power of all equipment. But here’s where many people make a mistake: don’t just add up the nameplate ratings of everything.
Apply these adjustments:
- Diversity factor: Not all equipment runs at the same time. A hotel kitchen doesn’t run every oven, grill, and dishwasher simultaneously. Apply a diversity factor of 0.7-0.8 for facilities with multiple similar loads.
- Actual load vs rated load: Most equipment doesn’t run at its rated capacity continuously. A 10 kW motor driving a pump might only draw 7 kW in actual operation. If possible, measure actual current draw with a clamp meter rather than relying on nameplate ratings.
Example running power calculation for a small hotel:
- Air conditioning (8 units × 3 kW running): 24 kW
- Lighting (all areas): 8 kW
- Kitchen equipment (diversity factor 0.7): 15 kW × 0.7 = 10.5 kW
- Water pumps (2 units × 5 kW): 10 kW
- Hot water heaters (2 units × 5 kW): 10 kW
- Office and security systems: 3 kW
- Total running load: 65.5 kW
Step 3: Account for Motor Starting Surges
This is the most critical — and most commonly overlooked — part of generator sizing. Electric motors draw 5-8 times their running current during the first few seconds of starting. This surge can overload a generator that’s perfectly sized for the running load.
For each motor, determine the starting method:
- DOL (Direct On Line): Starting current = 5-8 × running current. Used for small motors (typically under 7.5 kW). Highest surge demand.
- Star-Delta: Starting current = 2-3 × running current. Common for medium motors (7.5-37 kW). Reduces but doesn’t eliminate the surge.
- Soft Starter: Starting current = 2-3 × running current. Smooth ramp-up, minimal mechanical stress.
- VFD (Variable Frequency Drive): Starting current ≈ 1-1.5 × running current. Best for frequent starting applications.
The key rule: the generator must be able to handle the largest motor starting surge while maintaining voltage within ±15% (most equipment can tolerate this briefly). This means the generator’s capacity must be at least 1.5-2× the largest motor’s starting kVA.
In our hotel example, the largest motor is the air conditioning compressor at 3 kW (approximately 4 kVA running, 25-30 kVA starting with DOL). The generator needs to handle the 65.5 kW running load PLUS the starting surge of the largest motor.
Step 4: Apply the Sizing Formula
Here’s the formula we use at Higenset:
Required Generator Capacity (kVA) = (Total Running kW ÷ Power Factor) × 1.1 + Largest Motor Starting kVA × 0.5
The 1.1 factor provides a 10% safety margin. The 0.5 multiplier on motor starting kVA accounts for the fact that the generator’s voltage dip during motor starting is temporary — the generator’s overload capability handles the brief surge.
For our hotel example:
- Running load: 65.5 kW ÷ 0.8 = 82 kVA
- With 10% margin: 82 × 1.1 = 90 kVA
- Motor starting allowance: 30 kVA × 0.5 = 15 kVA
- Required capacity: 90 + 15 = 105 kVA
We’d recommend a 125-150 kVA generator for this application — rounding up to the next standard size and providing additional margin for future growth.
Step 5: Consider Future Expansion
One of the most common mistakes is sizing the generator for today’s needs without considering growth. We always ask our clients: “Where do you see your power demand in 3-5 years?”
For most projects, we recommend adding 20-25% capacity for future expansion. It costs relatively little to buy a slightly larger generator today, but retrofitting a larger unit later is expensive and disruptive.
In our hotel example, if the client plans to add 20 more rooms in two years, we’d factor in the additional air conditioning and amenity loads. A 150 kVA unit would provide comfortable headroom for both current and future needs.
Step 6: Verify with the Supplier
Once you’ve completed your calculations, share them with your generator supplier. A reputable supplier should review your load list and either confirm your sizing or recommend adjustments based on their experience with similar applications.
At Higenset, we provide free sizing consultations. We’ve sized generators for hundreds of projects across different industries and regions, and we can often spot issues that a straightforward calculation might miss — for example, harmonic distortion from a large number of VFDs that requires a larger alternator, or altitude derating for installations above 1000 meters.
Special Considerations
Altitude Derating
Generators lose approximately 3% capacity for every 300 meters above sea level. A 500 kVA generator at 1500 meters elevation effectively becomes a 410 kVA generator. Always account for altitude in your calculations.
Temperature Derating
Ambient temperatures above 40°C require derating as well. Most manufacturers specify a standard rating at 25°C or 30°C. For every degree above the standard, expect roughly 1-2% capacity reduction.
Nonlinear Loads
If your facility has a high proportion of VFDs, UPS systems, or other nonlinear loads (common in data centers, modern factories, and large commercial buildings), the alternator needs to be oversized to handle harmonic currents. As a rule of thumb, oversize the alternator by 20-30% for facilities where nonlinear loads exceed 25% of total capacity.
Parallel Operation
For large installations or applications requiring high reliability, consider multiple smaller generators in parallel rather than one large unit. This provides redundancy, allows load-matching for fuel efficiency, and simplifies maintenance (one unit can be taken offline while others continue operating).
Quick Reference: Common Application Sizing
Based on our experience, here are typical generator sizes for common applications:
- Small retail shop: 20-50 kVA
- Restaurant or cafe: 50-100 kVA
- Small hotel (30-50 rooms): 100-200 kVA
- Large hotel (100+ rooms): 300-800 kVA
- Office building (5-10 floors): 200-500 kVA
- Shopping mall: 500-2000 kVA
- Telecom base station: 15-50 kVA
- Small mining operation: 200-500 kVA
- Large mining operation: 500-2000+ kVA
- Hospital: 500-1500 kVA (with redundancy requirements)
- Data center: 500-5000+ kVA (with N+1 redundancy)
These are rough guidelines — actual sizing depends on specific equipment and operating conditions.
Conclusion
Proper generator sizing isn’t rocket science, but it does require a systematic approach. List your equipment, calculate running loads, account for motor starting surges, add margin for future growth, and verify with your supplier. The time you spend on sizing will pay for itself many times over in fuel savings, reduced maintenance, and reliable operation.
If you need help sizing a generator for your project, send us your equipment list and we’ll provide a detailed sizing calculation at no charge. We’ve sized generators for projects in over 30 countries, and we’re happy to put that experience to work for you.