Generator Sizing: How to Calculate Power Needs for Your Facility.

Choosing the right generator size is the single most important decision in your purchase. An undersized generator trips under load, shortens engine life, and damages sensitive equipment through voltage sags. An oversized generator wastes capital, increases fuel consumption per kWh, and suffers from wet stacking—a condition where unburned fuel accumulates in the exhaust system because the engine never reaches its optimal operating temperature. This guide provides the exact calculation method engineers use, with real-world examples and a downloadable approach.

Step 1: Identify Your Critical Loads

List every device or system that must run during a power outage. These are your critical loads. For a typical commercial facility, that includes lighting, HVAC, security, IT equipment, and process machinery. Categorize each load as resistive (heaters, incandescent lights), inductive (motors, transformers), or capacitive (UPS systems, power factor correction capacitors). Each type has different starting current characteristics.

Read the nameplate rating (in watts or amps) for each device. If only amps are available: Watts = Volts x Amps. For three-phase equipment: Watts = Volts x Amps x 1.732 x Power Factor.

Step 2: Calculate Running vs. Starting Power

You need two numbers that define your generator requirement:

• Running kW: Sum of all loads’ running watts, divided by 1000. This determines the continuous power output the generator must deliver.
• Starting kVA: The largest motor’s starting kVA (typically 4-6x its running kVA) plus the sum of all other loads’ running kVA. This determines the surge capacity the generator must handle during motor startup.

The generator must satisfy both numbers simultaneously. Example: A facility with 30kW running load and a 15kW motor with 6x starting surge requires a generator capable of 30kW continuous plus (15 x 6 – 15) = 75kVA starting overhead. A 50kW (62.5kVA) generator would handle this.

Step 3: Apply Safety Margins and Derating

Never size exactly to your calculated load. Industry best practice includes multiple safety margins:

• 25% future expansion margin — facilities invariably add equipment. Building this in now avoids costly generator replacement later.
• 10% altitude derating for every 1000m above sea level. At 2000m, a 500kW generator delivers only about 400kW.
• 10% temperature derating for ambient temperatures above 40C/104F. Desert installations in the Middle East commonly require this.
• 5% aging margin — engine output decreases 3-5% over the first 10,000 operating hours due to wear.

Final sizing formula: Generator Size (kVA) = (Total Running kW x 1.25) / 0.8. For a 100kW load: (100 x 1.25) / 0.8 = 156 kVA. Round up to the nearest standard size: 160 kVA or 200 kVA.

Common Sizing Mistakes to Avoid

• Ignoring motor starting current — motors draw 4-6x running current at startup. This is the #1 cause of generator undersizing.
• Using nameplate kVA instead of kW — the engine is rated in kW (real power), while the alternator is rated in kVA (apparent power). Both must be checked.
• Forgetting non-linear loads — UPS systems, VFDs, and LED drivers produce harmonic distortion that can cause generator voltage regulators to oscillate.
• Not sequencing motor starts — starting all motors simultaneously creates a massive starting surge. Staggering starts 5-10 seconds apart reduces peak demand significantly.
• Adding 100% redundancy unnecessarily — N+1 configuration for critical loads is standard, but 2N is only required for Tier IV data centers.

Sizing Examples by Application

For a detailed walkthrough with downloadable calculator, see our Generator Sizing Guide. For personalized sizing assistance, contact our engineering team with your load list.

FAQ

How do I size a generator for motor loads?

Multiply the largest motor’s running kVA by its starting multiplier (6x for DOL, 4x for star-delta, 2x for soft start, 1.5x for VFD) and add the sum of all other loads’ running kVA. The generator must handle both the running kW and the starting kVA simultaneously. Always start the largest motor first when possible.

What is the 80% rule for generator sizing?

Generators should operate at 70-80% of rated capacity for optimal fuel efficiency and engine longevity. Running below 30% load causes wet stacking, while sustained operation above 90% increases wear and fuel consumption disproportionately. Size your generator so the typical load falls within this 70-80% window.

Can I use an online generator sizing calculator?

Online calculators provide good initial estimates but should be validated by a licensed electrical engineer for critical applications. They often miss site-specific factors like harmonic loads, altitude, ambient temperature, and starting sequence optimization.

How much extra capacity should I plan for future expansion?

Industry standard recommends 25% spare capacity. This accommodates typical equipment additions over 5-10 years without requiring a generator upgrade. For rapidly growing facilities like data centers, consider 40% spare capacity or a modular paralleling system that can add generators incrementally.

What happens if my generator is oversized?

Oversized generators operating below 30% load suffer from wet stacking—unburned fuel and carbon deposits accumulate in the exhaust, reducing efficiency and engine life. Fuel consumption per kWh increases 15-25% compared to properly loaded operation. Regular load bank testing helps mitigate this.

Do I need to size differently for standby vs. prime power?

Yes. For standby applications, you can size closer to the maximum load since runtime is limited. For prime power, add extra margin because the generator runs continuously and needs headroom for load fluctuations. Prime-rated generators also require a 10% overload capability per ISO 8528.

How do harmonics from UPS and VFDs affect sizing?

Non-linear loads produce harmonic currents that cause additional heating in the generator alternator. As a rule of thumb, if non-linear loads exceed 25% of total load, increase the alternator size by 20-30% or specify a generator with a 3/3 pitch winding that naturally suppresses third harmonics.

What is the difference between single-phase and three-phase sizing?

Three-phase generators deliver 1.732x more power per amp than single-phase at the same voltage. A 100A, 400V three-phase generator provides about 69kW, while a 100A, 240V single-phase generator provides only 24kW. Most commercial and industrial applications use three-phase.

How do I account for starting sequence in sizing?

Always start the largest motor first when the rest of the loads are minimal. This reduces the peak starting kVA the generator must handle. If you must start motors simultaneously, add all their starting kVA values together—this typically requires a much larger generator than sequential starting.

Should I use a load bank to verify my generator sizing?

Absolutely. A load bank test after installation confirms the generator can handle its rated load and reveals issues like voltage dip during motor starting, frequency stability, and fuel consumption under actual conditions. Annual load bank testing is recommended for all standby generators.

Technical Deep Dive: Advanced Sizing Calculations

Motor Starting Methods and Their Impact

The method used to start motors has a dramatic impact on generator sizing. A 100kW motor can require anywhere from 150kW to 600kW of generator capacity depending on the starting method. Understanding these differences is critical for cost-effective generator sizing.

A practical example: A facility has three 50kW motors. With DOL starting, the generator must handle 3 x 50kW x 7 = 1050kVA starting surge (starting all three simultaneously). With soft starters, the surge drops to 3 x 50kW x 2.5 = 375kVA. With VFDs, it drops further to 3 x 50kW x 1.2 = 180kVA. The generator sizing difference: 1300kVA (DOL) vs. 500kVA (soft start) vs. 300kVA (VFD). The cost savings from specifying soft starters or VFDs can be $30,000-80,000 on the generator alone.

Non-Linear Load Considerations

UPS systems, VFDs, LED lighting drivers, and other switch-mode power supplies draw non-sinusoidal current that contains harmonic frequencies. These harmonics cause additional heating in the generator alternator, requiring derating of 20-40% unless the generator is specifically designed for non-linear loads. Key derating factors:

Industry-Specific Sizing Case Studies

Hospital Sizing Example (200 beds)

A 200-bed hospital has the following critical loads: ICU (80kW), operating rooms (40kW), emergency lighting (15kW), HVAC for critical areas (120kW), medical imaging (150kW starting, 60kW running), elevators (2 x 30kW, 150kW starting each), and IT/server room (25kW). Total running load: 390kW. Maximum starting surge (all motors starting simultaneously, unlikely but conservative): 390 + 150 + 2×120 = 780kVA. With sequential starting: 390 + 150 = 540kVA (first MRI), then 390 + 60 + 120 = 570kVA (elevators). Recommended: 800kVA generator (640kW) with PRP rating, providing 64% load factor at running load and adequate margin for starting surges.

Data Center Sizing Example (500kW IT load)

A Tier III data center with 500kW IT load, plus cooling (200kW), lighting and security (20kW). Total running load: 720kW. UPS systems with 30% THDi require 20% alternator derating. Recommended: 2 x 800kVA generators (N+1 configuration), each capable of carrying the full 720kW load at 90% load factor with derating applied. The N+1 configuration ensures uninterrupted power during generator maintenance or failure.

Mining Camp Sizing Example (1 MW)

A remote mining camp at 2500m altitude with 800kW running load (crushing plant, conveyors, camp facilities). Altitude derating at 2500m: approximately 22%. Temperature derating (45C ambient): additional 5%. Combined derating: 27%. Required generator capacity: 800kW / 0.73 = 1096kW. Recommended: 2 x 800kW generators in parallel (total 1600kW, derated to 1168kW), providing N+1 redundancy and 72% load factor with one unit running.

Additional FAQ

How do I size a generator for a facility with a large UPS?

The UPS presents two challenges: (1) The UPS battery charging load adds to the generator load after a power failure, typically 10-20% of the UPS rating. (2) The UPS rectifier draws non-linear current, requiring generator derating of 20-30%. Rule of thumb: Generator kVA = UPS kVA x 1.4 to 1.6 (assuming the UPS is the dominant load). For mixed loads, calculate the UPS contribution separately using the derated alternator capacity.

What is the minimum load I should put on a generator?

Most manufacturers recommend a minimum sustained load of 30% of the generator’s rated capacity. Below this threshold, combustion efficiency drops, carbon deposits accumulate, and wet stacking begins. If your facility’s minimum load is below 30%, consider: (1) Installing a smaller generator, (2) Using paralleling to shut down generators when load is low, (3) Installing an automatic load bank that activates below 30% load.

How does altitude derating work for generator sizing?

Above 1000m (3280ft), engine power output decreases approximately 3-4% per 500m due to reduced air density. At 2000m, a 1000kW generator delivers only about 860-880kW. At 3000m, it drops to about 740-760kW. Some turbocharged engines with altitude-compensating turbos can maintain rated power up to 2000-2500m. Always check the manufacturer’s altitude derating curve for the specific engine model.

Should I size my generator for peak demand or average demand?

Size for peak demand with a 25% margin. The generator must handle the highest load it will ever see, including motor starting surges. However, if your peak demand is significantly higher than average (e.g., a large motor that runs only occasionally), consider using soft starters or VFDs to reduce the starting surge rather than buying a much larger generator. Paralleling multiple smaller generators is another option for handling peak loads efficiently.

How do I account for future expansion in generator sizing?

Industry standard is 25% spare capacity. For rapidly growing facilities (data centers, manufacturing), consider 40% spare capacity or a modular paralleling system. The key insight: it is much cheaper to install a 25% larger generator now than to replace the entire generator in 3-5 years when you exceed its capacity. The incremental cost of a 625kVA vs. 500kVA generator is typically $8,000-12,000 (15-20%), while generator replacement costs $50,000-80,000 plus installation.

Huaquan Real-World Projects

At Huaquan Power, we’ve deployed generator systems across diverse applications worldwide. Here are some representative projects:

These real-world deployments demonstrate our engineering team’s capability to deliver reliable power solutions tailored to specific application requirements and environmental conditions. View all overseas case studies →

Recommended Generators by Power Range

Based on your power requirements, here are our recommended generator models:

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• 100kVA Generator Specs