What Size Volvo Genset Do You Need for Your Facility and How to Calculate It

Nothing frustrates facilities managers more than discovering their new generator can’t actually power their building. I’ve watched expensive generators sit idle during outages because someone guessed the size wrong—either massively oversized (wasting $30,000+) or dangerously undersized (failing when needed most).

After sizing hundreds of Volvo diesel generator set installations, I’ve learned that proper generator sizing isn’t rocket science, but it does require systematic thinking. Let me walk you through a practical framework for calculating exactly what kW capacity you need, avoiding the expensive mistakes that trip up first-time buyers.

Table of Contents

Understanding Power Ratings: Standby vs. Prime vs. Continuous

Before you calculate anything, understand that generators carry multiple power ratings, and using the wrong one causes problems.

Standby Power Rating

Definition: Maximum output the generator can supply for emergency backup use, assuming utility power is available most of the time.

Typical duty cycle: Up to 200 hours/year, with maximum runtime of 15 hours per outage.

Load factor: Assume 70-80% of rated capacity on average; occasional operation at 100% for brief periods (1-2 hours) is acceptable.

Applications: Emergency backup for facilities with reliable utility power—most commercial buildings, hospitals, data centers, manufacturing plants.

For a 200 kW standby-rated Volvo generator, you can pull 200 kW continuously for emergency use, but you shouldn’t run at this level 24/7/365. The engine is derated from its prime power capability to allow occasional overload and extended emergency operation without maintenance issues.

Prime Power Rating

Definition: Maximum continuous output available for unlimited hours per year when utility power is unavailable or when used as the primary power source.

Typical duty cycle: Up to 8,000+ hours/year with variable loading.

Load factor: Assume 70-80% of rated capacity average with excursions to 100-110% (10% overload capability typically available for 1 hour out of 12).

Applications: Remote sites without utility power, facilities with unreliable utility power requiring frequent generator operation, construction sites, mining operations.

A 200 kW prime-rated generator can supply 200 kW continuously, year-round. It can briefly handle 220 kW (10% overload), but shouldn’t regularly operate above rated capacity.

Continuous Power Rating

Definition: Constant maximum output available 24/7/365 at fixed load without variability.

Typical duty cycle: Baseload power generation, non-stop operation.

Load factor: 100% rated capacity continuously, no overload capability.

Applications: Continuous industrial processes, baseload utility generation, applications where load never varies significantly.

Continuous ratings are typically 80-90% of prime ratings. A genset rated 200 kW prime might be rated 180 kW continuous.

Step 1: Calculate Your Total Connected Load

Start by identifying every electrical device that might run simultaneously during generator operation. Don’t just guess—get real data.

Building-Wide vs. Critical Loads

Building-wide approach: Size the generator to replace utility power completely, running everything exactly as if utility power were available.

Advantage: No operational changes during outages; building functions normallyDisadvantage: Much larger (and more expensive) generator required

Critical loads approach: Identify truly essential systems and shed non-essential loads during generator operation.

Advantage: Smaller, less expensive generator; lower fuel consumptionDisadvantage: Requires load shedding planning; some building functions unavailable during outages

For most commercial facilities, the critical loads approach makes more sense. Hospitals, data centers, and manufacturing might need building-wide coverage to maintain operations.

Inventory Your Electrical Loads

Walk the facility with the electrical drawings and create a spreadsheet:

HVAC systems:

Chillers/air conditioning compressors
Air handler fans
Exhaust fans
Heating equipment (electric reheat, heat pumps)
Building pressurization fans
Combustion air fans (if gas heating)

Lighting:

Emergency and egress lighting (legally required)
General area lighting (separate essential from convenience lighting)
Exterior lighting (security vs. aesthetic)

IT and communications:

Server rooms and data centers
Network equipment rooms
Telephone systems
Security and access control systems
Fire alarm panels

Elevators:

Passenger elevators (how many must operate during emergency?)
Freight elevators
Wheelchair lifts

Kitchen and food service (if applicable):

Walk-in coolers and freezers
Refrigerators
Cooking equipment (often gas, but exhaust fans require power)
Dishwashers

Medical/industrial equipment (if applicable):

Critical manufacturing processes
Medical equipment (life support, imaging, laboratory)
Refrigeration for medical supplies or sensitive materials

Building support systems:

Water pumps (domestic water, fire suppression)
Sump pumps and drainage
Fuel pumps (for generator fuel transfer)
Battery chargers (emergency lighting, UPS systems)

For each item, note the name rating (from the equipment nameplate) and whether it runs continuously or intermittently.

Example: Medium Office Building (50,000 sq ft)

Let’s work through a real example to illustrate the process:

SystemEquipmentNameplate (kW)QuantityTotal (kW)HVACRooftop unit fans7.5430HVACExhaust fans2.2613.2LightingEmergency/egress0.3100 fixtures30LightingCritical area (LED)0.05200 fixtures10ITServer room AC5210ITNetwork/telecom–8ElevatorsPassenger elevator25250PumpsDomestic water5.515.5MiscReceptacle loads–15TOTAL CONNECTED LOAD171.7 kW

Now, does this mean we need a 172 kW generator? Absolutely not. We haven’t yet applied demand factors and motor starting requirements.

Step 2: Apply Demand Factors

Not every connected load operates at full capacity simultaneously. Demand factors account for real-world diversity in electrical loads.

HVAC Demand Factors

Air conditioning and heating rarely run at full capacity except during extreme weather. Typical demand factors:

Air handler fans: 0.8-1.0 (run continuously but rarely at full speed with VFD control)
Compressors/chillers: 0.6-0.8 (cycling based on load; rarely all units run simultaneously at peak)
Exhaust fans: 0.5-0.7 (intermittent operation in most facilities)

Lighting Demand Factors

Emergency lighting: 1.0 (legally required; must operate at full capacity)
General area lighting: 0.6-0.8 (not all areas occupied simultaneously; some circuits can be shed)
Exterior lighting: 0.5 (much can be shed except security lighting)

Receptacle and Small Equipment Loads

General office receptacles: 0.4-0.5 (diversity of computers, printers, chargers, etc.)
Kitchen/break room: 0.3-0.5 (rarely use all appliances simultaneously)
Medical/lab equipment: 0.6-0.8 (higher diversity for general equipment; 1.0 for life-critical devices)

Elevator Demand Factors

This is tricky because elevators are high-power intermittent loads.

Single elevator: 1.0 (it’s either running or not)
Multiple elevators: 0.5-0.7 (unlikely all elevators run simultaneously in same direction at full load)

For emergency planning, many engineers calculate for one elevator per 4-5 floors operating, rather than all elevators.

Applying Demand to Our Example

Let’s revise our office building calculation:

SystemConnected (kW)Demand FactorDemand (kW)HVAC – Fans300.8525.5HVAC – Exhaust13.20.67.9Emergency lighting301.030General lighting100.77Server room AC101.010IT/telecom81.08Elevators (1 of 2)500.525Water pump5.50.84.4Receptacles150.46TOTAL DEMAND LOAD171.7123.8 kW

We’ve reduced our connected load of 172 kW down to a demand load of 124 kW—nearly 30% reduction. But we’re not done yet.

Step 3: Account for Motor Starting Currents

Electric motors draw 4-8× their running current during startup—a phenomenon called inrush current or locked rotor current. If you don’t size the generator for motor starting, it will stall or trip offline when large motors start.

Understanding Motor Starting Loads

When a three-phase induction motor starts, it initially acts like a short circuit until the rotor begins spinning. Typical starting current multipliers:

Across-the-line starters (full voltage): 6-8× running current
Reduced voltage starters (wye-delta, autotransformer): 3-5× running current
Soft starters (electronic reduced voltage): 2-3× running current
Variable frequency drives (VFD): 1.2-1.5× running current (minimal inrush)

The generator must provide enough instantaneous power to start the motor without voltage sag exceeding 15-20% (acceptable for most loads) or 10% (for sensitive electronics).

Motor Starting Calculation Methods

Simple method (conservative): Multiply the largest motor’s full-load kW by 3-6× (depending on starter type) and add it to your base running load.

Example: Our office building has two 25 kW elevators as the largest motors. Assume across-the-line starting:

Running base load (excluding elevators): 98.8 kW
Elevator starting load: 25 kW × 6 = 150 kW
Total starting requirement: 98.8 + 150 = 248.8 kW

That’s double our running demand! This illustrates why motor starting often drives generator sizing.

Refined method: Calculate kVA rather than kW for motor starting, since starting power factor is very poor (0.2-0.3). Generators are often sized by kVA capacity during starting.

For our 25 kW elevator motor:

Running: 25 kW at 0.85 power factor = 29.4 kVA
Starting: 25 kW × 6 = 150 kW at 0.25 power factor = 600 kVA (!)

Most generator manufacturers provide motor starting charts showing what size motor (in HP or kW) each generator model can start. This is often your sizing constraint.

Sequenced Starting and Load Management

Rather than accepting massive generators sized for worst-case starting, smart designs use sequenced motor starting:

Generator starts and stabilizes at base load (lights, HVAC fans, IT)
Controller waits 10-30 seconds for voltage/frequency stabilization
First large motor starts (elevator 1)
Controller waits 5-10 seconds for recovery
Second large motor starts (elevator 2)

With sequenced starting, you might size for base load + largest motor starting, rather than base load + all motors starting simultaneously. This can reduce required generator size by 30-40% compared to simultaneous starting.

Companies like Tesla Power typically include sequenced starting logic in their automatic transfer switch and control packages, making this straightforward to implement.

Step 4: Add Safety Factor and Future Growth

Never size a generator exactly to your calculated load. Build in margin for:

Safety Factor

Standard practice adds 10-25% safety margin above calculated demand:

10-15%: Well-defined loads with little variability (data centers, industrial processes)
15-20%: Typical commercial buildings with moderate load variability
20-25%: High uncertainty about loads, poor load documentation, or highly variable processes

Our office building: 124 kW demand × 1.2 safety factor = 148.8 kW minimum

Future Growth

Will your facility expand? Add equipment? Increase occupancy? Think 5-10 years ahead:

0-10%: Mature facility unlikely to expand
10-20%: Typical growth allowance for commercial facilities
20-30%: Rapid growth anticipated or major expansion planned

For a growing company, adding 15% growth allowance: 148.8 kW × 1.15 = 171.1 kW

Optimal Generator Loading

Diesel generators operate most efficiently and reliably at 60-80% load. Running continuously below 30% load causes “wet stacking” (unburned fuel accumulates in the exhaust system) and increases maintenance. Running continuously above 85% load reduces engine life and leaves no headroom for transients.

For our office building calculation showing 171 kW requirement, optimal sizing would target this as 70% of generator capacity:

171 kW ÷ 0.70 = 244 kW generator capacity

This provides comfortable headroom for motor starting, future growth, and optimal engine operation.

Step 5: Select Standard Generator Size

Generators come in standard sizes. For Volvo Penta-powered units, common standby power generator sizes include:

Small commercial: 20, 30, 40, 50, 60, 80, 100 kWMid-size commercial: 125, 150, 175, 200, 250, 300, 350, 400 kWLarge industrial: 450, 500, 600, 750, 800, 1000, 1250, 1500, 2000 kW

For our example requiring 244 kW, the next standard size up is 250 kW or 300 kW.

250 kW choice: Tighter fit; 171 kW demand = 68% loading (good); less expensive; adequate for most scenarios.

300 kW choice: More margin; 171 kW demand = 57% loading (comfortable); more room for growth; better motor starting capability; higher cost but better long-term flexibility.

For critical facilities, I usually recommend the larger size. For non-critical commercial, the 250 kW is adequate. Cost difference might be $5,000-8,000—cheap insurance for a 20-year asset.

Special Considerations for Specific Applications

Data Centers and IT Facilities

Power quality requirements: IT equipment needs better voltage regulation (±1-2%). Consider UPS systems to bridge generator startup time (10-15 seconds), static transfer switches for ultra-fast switching, and redundant generators (N+1 or 2N configuration).

Cooling loads: Server room cooling is your largest continuous load. Air conditioning compressors have large starting currents—soft starters or VFDs strongly recommended.

Growth trajectory: Data centers expand rapidly. Oversize 30-50% beyond current requirements or plan modular expansion with parallel generators.

Hospitals and Healthcare Facilities

Life safety codes: NFPA 99 and Joint Commission standards mandate emergency generator auto-start within 10 seconds, with certain loads (life support, surgical lights) transferring in <10 seconds. Generator must support 96-hour runtime (fuel capacity requirement).

Load classification: Life safety (immediate), Critical (10-second transfer), Equipment (delay permitted). Separate transfer switches for each class with sequenced connection prevent overloading.

Sizing approach: Hospitals typically size for 60-80% of total building load; non-critical areas operate in reduced mode during extended outages.

Manufacturing and Industrial Facilities

Process criticality: Continuous processes (chemical plants, food processing) require full capacity prime power rating generators. Batch processes can often pause between batches, allowing smaller standby power generators.

Large single motors: Industrial facilities often have 50-200 HP motors (37-150 kW) that dominate sizing calculations. VFD-controlled starting reduces inrush to manageable levels; soft-start systems reduce starting current to 2-3× running current.

Remote Sites and Off-Grid Applications

Prime power rating required: Use prime power or continuous power ratings for generators running continuously or near-continuously. No utility backup means higher duty requirements.

Fuel storage: Size for minimum 7-day runtime. Remote locations (offshore platforms, Arctic stations) often require 30+ days capacity, dramatically impacting fuel system costs.

Paralleling and redundancy: Specify multiple smaller generators over one large unit for N+1 redundancy, maintenance flexibility, and better load-following efficiency.

Common Sizing Mistakes to Avoid

Mistake #1: Using Total Breaker Capacity – Breakers are sized for wire protection, not actual load. Use actual connected equipment nameplates, not the sum of all breaker ratings.

Mistake #2: Ignoring Power Factor – Motors and transformers have poor power factor (0.7-0.8), drawing more apparent power (kVA) than real power (kW). Generators are limited by both kW and kVA capacity.

Mistake #3: Single Snapshot Load Measurement – Measure or calculate for worst-case conditions: hottest/coldest day, maximum occupancy, peak activity times.

Mistake #4: Undersizing for Cost Savings – Don’t save $5,000 upfront to risk failure during critical outages. The long-term regret isn’t worth it.

Mistake #5: Oversizing Excessively – A 500 kW generator for 150 kW load causes poor fuel efficiency, wet stacking, and unnecessary maintenance costs. Target 60-80% loading.

Working with Suppliers on Generator Sizing

Don’t size your generator alone—leverage supplier expertise. Companies like Tesla Power have engineers who do this daily and can:

Working with Suppliers on Generator Sizing

Leverage supplier expertise. Companies like Tesla Power have engineers who perform load calculations, recommend motor starting approaches, provide load bank test reports, advise on growth, and quote multiple options (good/better/best sizing). Get sizing calculations in writing—if recommended capacity proves inadequate, you need documentation. Reputable suppliers provide formal load calculation reports as part of quotation packages.

Quick Sizing Rules of Thumb

For ballpark estimates before detailed calculations:

Example: 30,000 sq ft office × 12 W/sq ft = 360 kW full building. For critical loads only, reduce 40-60% = 144-216 kW range. These are rough estimates only.

Frequently Asked Questions

Q: Can I parallel multiple smaller generators instead of buying one large unit?

Yes, and this often makes sense for larger installations (400 kW+) or when redundancy is critical. Paralleling two 200 kW generators gives you 400 kW capacity plus redundancy if one unit fails. Control systems (ComAp, Deep Sea Electronics) handle load sharing automatically. However, paralleling adds complexity and cost—typically worthwhile only for 300+ kW installations or N+1 redundancy requirements. Companies like Tesla Power can engineer complete paralleling systems including synchronizing switchgear.

Q: Should I size for standby or prime power rating?

Use standby power rating if utility power is generally reliable and generator runs only during outages (most commercial buildings). Use prime power rating if generator runs frequently or continuously (remote sites, unreliable utility, frequent long-duration outages). Prime-rated generators typically cost 10-15% more than equivalent standby models and have different maintenance schedules. Don’t use standby-rated generators for frequent or continuous operation—you’ll dramatically shorten engine life.

Q: What if my load calculations show I need an unusual size like 385 kW?

Generators come in standard sizes. Round up to the next standard size (400 kW in this case). The 15 kW “extra” capacity provides margin for future growth and motor starting. Don’t try to custom-order odd sizes—standard models have better availability, lower cost, easier parts sourcing, and better resale value. The premium for standard sizes over “exact” sizing is always worthwhile.

Q: How often should I test my generator to verify sizing is adequate?

Monthly testing at 30% load minimum (NFPA requirement) confirms operability but doesn’t validate full-capacity sizing. Annual load bank testing at 100% of rated load for 1-2 hours confirms your generator can handle maximum design load and prevents wet stacking. For critical facilities, perform full-load testing (actually running the facility on generator power) annually during scheduled maintenance windows—this is the only true validation of your sizing calculations under real-world conditions.

Q: What’s the difference between kW and kVA, and which one matters for generator sizing?

kW (kilowatts) measures real power—the actual work being done. kVA (kilovolt-amperes) measures apparent power, which includes reactive power from inductive loads (motors, transformers). The relationship is kW = kVA × power factor. Generators have both kW limits (engine capacity) and kVA limits (alternator capacity). For facilities with many motors (low power factor 0.7-0.8), you might hit kVA limits before kW limits. Always specify your load’s power factor when discussing sizing with suppliers—assuming 0.8 power factor is standard, but verify for your specific application.

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