Commercial Generator Sizing Calculator

Total Continuous Load (kW): Sum of all equipment running continuously (e.g., lights, HVAC, computers).

Overall System Power Factor: Typically between 0.8 (inductive loads) and 1.0 (resistive loads).

Largest Motor Running Power (kW): The running power of the single largest motor or inductive load.

Largest Motor Starting kVA Multiple: How many times its running kVA is its starting kVA (e.g., 6 for Direct-On-Line start).

Desired Generator Load Factor (%): Recommended 80% for continuous operation to ensure longevity and efficiency.

Required Generator Size:

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function calculateGeneratorSize() { var totalContinuousLoadKW = parseFloat(document.getElementById('totalContinuousLoadKW').value); var overallPowerFactor = parseFloat(document.getElementById('overallPowerFactor').value); var largestMotorKW = parseFloat(document.getElementById('largestMotorKW').value); var motorStartingMultiple = parseFloat(document.getElementById('motorStartingMultiple').value); var generatorLoadFactor = parseFloat(document.getElementById('generatorLoadFactor').value); // Input validation if (isNaN(totalContinuousLoadKW) || totalContinuousLoadKW < 0 || isNaN(overallPowerFactor) || overallPowerFactor 1 || isNaN(largestMotorKW) || largestMotorKW < 0 || isNaN(motorStartingMultiple) || motorStartingMultiple < 1 || isNaN(generatorLoadFactor) || generatorLoadFactor 100) { document.getElementById('requiredKVA').innerHTML = 'Please enter valid positive numbers for all fields.'; document.getElementById('requiredKW').innerHTML = "; return; } // Step 1: Calculate Total Running kVA var totalRunningKVA = totalContinuousLoadKW / overallPowerFactor; // Step 2: Calculate Largest Motor Running kVA var largestMotorRunningKVA = largestMotorKW / overallPowerFactor; // Step 3: Calculate Largest Motor Starting kVA var largestMotorStartingKVA = largestMotorRunningKVA * motorStartingMultiple; // Step 4: Determine Transient kVA Demand (when largest motor starts) // This is the sum of all other running loads plus the starting kVA of the largest motor. var transientKVADemand = (totalRunningKVA – largestMotorRunningKVA) + largestMotorStartingKVA; // Step 5: Determine the Peak kVA Requirement // The generator must handle either the total continuous running kVA or the transient kVA demand, whichever is higher. var peakKVADemand = Math.max(totalRunningKVA, transientKVADemand); // Step 6: Apply Generator Load Factor // To ensure longevity and efficiency, generators are often not run at 100% capacity. var requiredGeneratorKVA = peakKVADemand / (generatorLoadFactor / 100); // Step 7: Convert Required Generator kVA back to kW (for reference) var requiredGeneratorKW = requiredGeneratorKVA * overallPowerFactor; document.getElementById('requiredKVA').innerHTML = '' + requiredGeneratorKVA.toFixed(2) + ' kVA'; document.getElementById('requiredKW').innerHTML = '(' + requiredGeneratorKW.toFixed(2) + ' kW)'; }

Understanding Commercial Generator Sizing

Properly sizing a commercial generator is a critical step in ensuring reliable backup power for your business or facility. An undersized generator won't be able to handle your load requirements, leading to shutdowns and potential damage, while an oversized generator is an unnecessary expense, less fuel-efficient, and can suffer from "wet stacking" due to under-loading.

What is Generator Sizing?

Generator sizing involves calculating the total electrical load your facility requires during a power outage and selecting a generator with sufficient capacity to meet that demand. This isn't just about summing up the running power of all your equipment; it also involves accounting for specific types of loads, especially motors, and applying safety factors.

Key Factors in Sizing a Commercial Generator

Several crucial elements influence the final generator size:

1. Total Continuous Load (kW)

This is the sum of all electrical power (in kilowatts) consumed by equipment that will operate simultaneously during a power outage. This includes lighting, HVAC systems, computers, servers, refrigeration, and other essential machinery. It's vital to conduct a thorough load audit to identify all critical loads.

2. Power Factor

Power factor is a measure of how effectively electrical power is being used. It's the ratio of real power (kW) to apparent power (kVA). Inductive loads, like motors and fluorescent lights, cause the current and voltage waveforms to be out of phase, resulting in a power factor less than 1.0. Generators are rated in kVA because they must supply both real power (kW) and reactive power (kVAR). A typical commercial power factor is around 0.8.

3. Motor Starting Current (Inrush)

Motors, especially large ones, require a significantly higher amount of current to start than they do to run continuously. This momentary surge, known as inrush current or locked rotor amps (LRA), can be 3 to 10 times their normal running current. The generator must be capable of supplying this high starting current without experiencing a significant voltage drop that could cause other equipment to malfunction or trip. Our calculator uses a "Starting kVA Multiple" to account for this.

4. Desired Generator Load Factor (Derating)

It's generally recommended not to run a generator at 100% of its rated capacity for extended periods. Operating at 70-80% of its capacity allows for better fuel efficiency, extends the generator's lifespan, provides a buffer for unexpected load increases, and reduces wear and tear. This is why a load factor (or derating factor) is applied.

kW vs. kVA: What's the Difference?

kW (kilowatt): Represents the "real power" or actual power consumed by the load and converted into useful work (e.g., heat, light, mechanical motion).
kVA (kilovolt-ampere): Represents the "apparent power," which is the total power supplied by the generator. It's the vector sum of real power (kW) and reactive power (kVAR). Generators are typically rated in kVA because they must supply both.

The relationship is: kVA = kW / Power Factor. For purely resistive loads (like incandescent lights or heaters), the power factor is 1.0, so kW = kVA. For inductive loads, kVA will always be greater than kW.

How to Use This Calculator

Total Continuous Load (kW): Enter the total running power of all equipment that needs to operate during an outage.
Overall System Power Factor: Input the estimated power factor for your facility. If unsure, 0.8 is a common conservative estimate for mixed commercial loads.
Largest Motor Running Power (kW): Identify the single largest motor or inductive load in your facility and enter its running power.
Largest Motor Starting kVA Multiple: This value indicates how many times its running kVA the largest motor requires for starting. A common value for Direct-On-Line (DOL) starting is 6. Consult motor specifications or an electrician for precise values.
Desired Generator Load Factor (%): Enter the percentage of the generator's capacity you wish to utilize. 80% is a widely recommended practice.
Click "Calculate Generator Size" to get your recommended kVA and kW ratings.

Interpreting Your Results

The calculator will provide two key outputs:

Required Generator kVA: This is the primary rating you should look for when selecting a generator. It accounts for both continuous running loads and the transient demands of motor starting, plus your desired load factor.
Required Generator kW: This is the real power component of the required kVA, provided for reference.

Always round up to the next standard generator size available from manufacturers. For example, if the calculator suggests 210 kVA, you might look for a 220 kVA or 250 kVA generator.

Disclaimer: This calculator provides an estimate for preliminary planning. For precise generator sizing and selection, always consult with a qualified electrical engineer or generator specialist. They can perform a detailed load analysis, consider specific site conditions, harmonic distortion, altitude, temperature derating, and local electrical codes.