Industrial Generator Sizing: A Step-by-Step Guide and Calculator

You've been researching and shopping for what seems to be the ideal industrial generator for weeks. The wattage matches, budget is approved, and delivery date is confirmed. However, once the primary grid fails, all your big motors attempt to start up at the same time and blow the circuit. Lights out, machines stop, and the "perfect" generator becomes an expensive brick.

If you're reading this, then you want to avoid that scenario at all costs.

When sizing an industrial generator it is not as easy as just reading the nameplate on your main breaker or blindly adding 20% to your total wattage. It is a problem to be solved by engineering. There are hidden starting surges, complex power factors, and site conditions (such as high ambient temperatures in the summer or altitude) that must be taken into consideration that reduce the true output of the engine.

We'll take you through the exact steps to correctly size an industrial generator in this guide. Together we'll create a load schedule, determine the actual electrical usage, and simulate those "surprises" that many buyers encounter when they see what happens when a motor starts. Let’s get to work.

Quick Answer: How to Size an Industrial Generator

Determining the correct size industrial generator requires first determining the operating duty (standby or prime) and the specific load(s) to be served. Then, construct a detailed load schedule and determine the maximum running kW and kVA. Next, test the heaviest transient conditions (e.g. large motor starts or motor load steps) and verify that the alternator can withstand the voltage dip. Lastly, compare actual candidate models with derating factors for site-specific temperature and altitude.

Step 1: Define the Generator Duty and Loads to Support

You have to establish the job first before you examine any one number. What is the purpose of this generator hire?

Is it just sitting in a corner waiting to save your Data Center in the event of a utility blackout? Or is it operating a far-out mining operation in the desert, working around the clock for no other energy supply? If the overall wattage seems the same, these situations call for entirely different machines.

Standby, Prime, and Continuous Are Different Rating Conditions

Generator ratings are not simply marketing jargon, but are hard limits set by international standards such as ISO 8528.¹

• Standby Rating: This is for emergency back up. The generator is intended for use for a limited number of hours per year, with varying loads, during utility outages.
• Prime Rating: This is the primary power source in the absence of a utility grid. It can operate for any number of hours per year but generally with a varying load level with an average set for a percentage over time.
• Heavy lifter is Continuous Rating. It provides a fixed, non-varying 100-percent load, for an infinite number of hours.

Before you go memorizing those descriptions, one caveat: ISO sets the framework, and each manufacturer imposes its own sub-categories, hours and average-load rules. So use the categories above as the form of the problem, not as the solution to your unit. The only numbers that matter are those on the data sheet for the exact model you are purchasing.

You can't take a 500 kW standby generator and load it with a 500 kW constant load at a far away site without over heating the alternator, stressing the engine and the warranty. You have to be realistic about your duty rating.

Full-Facility, Essential-Load, and Staged-Restart Scopes

It is also important to consider: When power is lost, does everything have to start up simultaneously?

If you're backing up a hospital or a critical data center, the answer may be yes. For a manufacturing plant, however, it may be sufficient to maintain only the security systems, information technology, and emergency lighting, while the large production lines are left off.

Classify your loads into CRITICAL (must run) and DEFERRAble (can wait, or can be dropped altogether). If you start your generators in a staggered manner (largest motors first and then smaller loads), you can significantly decrease the size (and cost) of the generator required.

Step 2: Build an Industrial Load Schedule

Now we need data. The load schedule is the base of your whole task of sizing. Otherwise you are only guessing.

As Schneider Electric's installation guide explains, "installed power" (the nameplate rating on the equipment) is not the same as "simultaneous demand" (the power that the generator must supply at any one time).²

List Every Load That May Be on the Generator

Take a walk through your facility or blueprints, and make a list of everything. Motors, pumps, air compressors, HVAC chillers, lighting banks, IT UPS systems, heating elements, and cyclic loads such as welders. Never underestimate the importance of the little things—they add up.

As you go through the list, label each row as critical or deferrable. Ten seconds is all it takes now, and a week later you are trying to figure out what loads you may be able to drop to allow a large motor to start.

Capture Electrical and Operational Fields

For each item of equipment you have on your list, you must collect the following:

• Quantity
• Voltage (V), Phase and Frequency (Hz)
• Mechanical Output (HP) or Electrical Input (kW)
• Efficiency and Power Factor (PF)
• Running Current
• Locked-Rotor / Starting Current
• Starting Method (Direct-on-line, Soft Starter, VFD)
• Load Step: the amount that this device loads onto the system when it powers on or switches on.
• Operating Hours and Diversity: how long it's operating, and if it is really operating at the same time as everything else

Write down the source of each figure and if it is confirmed. Three different levels of certainty exist - a nameplate reading, a manufacturer datasheet, and an operator's best guess - and your completed sizing is only as good as the worst number in this column.

Separate Current and Future Loads

Don't be coy about your growth plans. Maintain three columns: lowest load you currently run, highest load you currently run and actual growth you have committed to – dates and equipment. When you know you will be adding a second production line next year, plan for it now! But don't add a buffer for growth that no one has authorized, say, 30% "just in case".

Step 3: Calculate Running kW and kVA Demand

So let's do some calculations. These pumps, lights, chillers, etc. have to be converted to the two numbers that the generator is interested in: Real Power (kW) and Apparent Power (kVA).

Convert HP or Motor Shaft kW to Electrical Input

A motor marked "100 HP" does 100 HP of mechanical work at the shaft. It is NOT the electrical power it consumes from the generator. The electrical input requires consideration of the efficiency of the motor.

Only mechanical units can be converted by the standard conversion (1 HP = 0.746 kW).³ To determine the actual generator perception:

Motor Input kW = (HP × 0.746) ÷ Efficiency⁴

When efficiency is not provided, a typical value for large industrial motors is 90-93%.

Convert kW to kVA Using the Operating Power Factor

Not all electricity through wires is useful. Inductive loads (such as motors, transformers, etc.) cause the current to lag the voltage. The Power Factor (PF) is used to measure this.

The apparent power that the alternator would have to provide is:

kVA = kW ÷ PF⁵

If your load is 100KW with a terrible PF of 0.70, your alternator will need to provide approximately 142KVA. Assuming everything works at a perfect 1.0 PF will result in a significant undersizing of the alternator, which will cause it to overheat. Ideally, use a measured aggregate PF for your whole system and if working from estimates, state the PF for each load and how you arrived at the total, as you cannot simply add the per row kVA's together and say that this is the exact kVA of system.

Three-Phase Formula When Voltage and Current Are Known

The kVA can be calculated directly if the balanced line-to-line voltage (V_LL) and line current (I_L) is known:

kVA = √3 × V_LL × I_L ÷ 1000⁵

Note: If you already know the total kW, and the system PF, then you don't need to multiply by √3 again, just divide the kW by the PF.

This can be a stumbling block for many, and if you prefer to have the arithmetic on its own, check out our guide kW to kVA.

Step 4: Check Motor Starting and Load Steps

Most sizing projects fail here. The running kW looks good on paper but when the chiller starts up, the generator chokes. Why? Motors require a large start-up current to start, typically 6-8 times the operating current.⁴

Cummins technical whitepapers place a strong emphasis on the fact that sizing is not simply about wattage but about the whole generator system's ability to respond to and recover from such violent load steps.⁶

Identify the Largest Starting Event, Not Just the Largest Motor

Your largest motor may not be the most challenging test for your generator. It's the biggest starting event.

If the power is restored and a three medium-sized pumps start at the same time, the collective start-up event may be worse than your main chiller. Also, it is far more difficult to start a large motor with the generator operating at 70% capacity versus starting it when the generator is empty. On an automatic transfer switch, the restart sequence is the programmed sequence of the controls; it may not be what you expected.

Compare Direct-On-Line, Soft Starter, and VFD Starts

The starting of the motor is of great importance.

• Direct-On-Line (DOL): The most brutal way. Please be aware of a very high starting kVA (skVA) load.
• Soft Starters: These minimize the initial inrush current, which helps to minimize the mechanical and electrical shock.
• Variable Frequency Drives (VFDs): VFDs can virtually eliminate the starting surge. A VFD is not a “zero impact” device.

Before deciding, obtain four items from the equipment documentation, not memory: the starting current, the starting torque (if the load is difficult to turn, a soft starter could also reduce the torque), the harmonic profile of any drive, and the load inertia that you want to accelerate. Two pumps with the same nameplate kW can start very differently, one being a high inertia fan and the other a low inertia pump.

Build a Starting Sequence Table

To do this properly, sketch your sequence. To determine Step 1, enter the base running load, followed by the starting kVA of the first device. Look at the maximum allowable voltage dip (say, 15%). When that motor is operating, proceed with Step 2. Continue stacking the loads until they are all online.

Construct a table that has one column for the generator running load, one for the device to be installed, one for the device start-up power, one for the permissible dip time and recovery time, and one for the type of device start-up control. The number that sizes your alternator is the worst row in that table.

Consider Sensitive and Cyclic Loads

Not all equipment is able to handle voltage dips. Medical imaging machines, UPS systems, and PLCs will simply shut down when the voltage or frequency during a load step drops too far. The same goes for cyclic loads such as welders and large compressors; they return to your generator time after time throughout the day. The generator should be sized to ensure that the transient dip does not exceed the most sensitive equipment on the site—it's not the average of the equipment.

Step 5: Apply Duty Ratings, Site Conditions, and Minimum-Load Checks

Here are your numbers! Now, it's time to check the behavior of a real-world generator in your environment. The "500 kW generator" is not necessarily a 500 kW generator.

Compare the Correct Standby, Prime, or Continuous Rating

Don't take the standby maximum output for granted and think you can operate it for three weeks during a grid failure. Compare your demand to the particular duty rating you are looking to use, at the same voltage and frequency you will be using it.

Apply Model-Specific Temperature and Altitude Derating

Engines require high density, cool air for efficient fuel burn. Alternators require cool air to dissipate heat. The generator simply can't breathe if it is on a site 2,000 meters above sea level in the Andes or in 45°C ambient heat in the Middle East.

The engine and alternator will be derated (capacity reduced) and not necessarily by the same amount – one may be the limiting factor. Specific manufacturer derating curves for the specific model being considered must be consulted.⁷ Altitude penalties can vary between brands; a 10% altitude penalty for one brand could be a 15% altitude penalty for another brand.

Check Minimum Operating Load

Empty running is a no-no for diesel engines. When a large generator is sized for future growth but is only used for 15% of its capacity today, "wet stacking" may occur, where unburned fuel and soot collect in the system and ruin efficiency and cause severe damage to the engine.⁸

Step 6: Compare Candidate Generator Sets on One Specification Sheet

How to make a fair comparison when you get quotes from Caterpillar, Cummins or MTU? You put them on a one ruthless comparison sheet.

Pass/Fail Checks Before Price

Prior to examining the dollar amount, ensure that both machines actually pass engineering test:

• Are they operating at the same target voltage, frequency and duty? A figure taken from a standby column (60 Hz) is not comparable with a prime figure (50 Hz).
• Are they able to supply the continuous kW and kVA you need at the highest elevation, hottest ambient temperature at your site?
• Will both maintain the voltage dip within your voltage dip specifications during your worst-case motor start or load step?
• Are both above their minimum required load during your normal running hours?

If the voltage dip test is not passed, then a lower cost model is not a bargain, it is a liability.

Worked Example: Screen a Motor-Heavy Industrial Load

Let's apply this to a sample factory and observe where the process typically breaks down.

Establish the Running Baseline

Suppose that when we audit a facility, we determine that the absolute maximum simultaneous electrical input needed is 180kW. We use a power quality meter and discover that the overall system power factor (PF) is a rather poor 0.85.

Using our formula:

kVA = 180 kW ÷ 0.85 = 211.8 kVA

Three-phase power with a balanced line current is approximately 255 Amps if the facility is operating on 480 V. These two numbers represent: 180 kW is the total electrical input of all of the above operating simultaneously, and 0.85 is a measured system-level power factor – not an educated guess.

Identify the Worst Starting Step

Now, we know that the running baseline is 180kW / 211.8kVA. However, there is a large water pump at the factory that turns on/off when the rest of the factory is in operation.

In order to size the generator, we must know:

• What is the starting kVA of that pump?
• Is it Direct-On-Line or through a Soft Starter?

What to Send for an Industrial Generator Sizing Review

When requesting a supplier for a quote, avoid asking for "a 500 kW generator." Send them a complete Request for Quotation (RFQ) data pack.

Minimum RFQ Data Pack

Provide the supplier with the complete story:

• Destination and Site: Target market, delivery location, site altitude, maximum and minimum ambient temperatures.
• Duty Required: Standby, Prime, Continuous.
• Electrical Specs: Target Voltage/Target Phase/Target Frequency (Hz).
• Maximum and minimum expected running loads (kW and PF) - Load Profile.
• The Load Schedule, particularly emphasising motor starting sequences.
• Performance Limits: These are the maximum allowable voltage and frequency dip.
• Fuel type: Diesel / Natural gas / Dual fuel.
• Future Expansion: Any growth targets that are future dated and quantified that you wish this unit to cover.
• Compliance and Documentation: Required emissions standards (e.g., Tier 4 Final, Stage V), plus the certificates.

Industrial Generator Sizing FAQs

How Do You Calculate What Size Industrial Generator You Need?

The steps above are done in sequence. Create a complete load schedule to determine the maximum kW and kVA, motor start sequencing to determine transient surge, environmental derating, minimum load and then compare to the manufacturer's specific standby, prime or continuous ratings.

Can I Size a Generator From Total kW Alone?

No, not to a complete answer. Total kW does not account for the apparent power (kVA) that your power factor implies, does not address the surges needed for motor starts and cyclic peak loads, and provides no information on the duty rating you plan to use, or the altitude and temperature of your site.

What Is the Difference Between a 200 kW Standby and Prime Generator?

Don't read the two ratings as two versions of the same output, but as two classes of machine. 200 kW standby rating is short emergency duty with a specified hour limit and prime rating is continuous operation with a fluctuating average load. Both numbers are only significant when considered with the model's datasheet which will specify the model's running hours, average load rules and overload capacity.

How Do I Size a Three-Phase Generator?

Just as in any other supply, commence from the load schedule by mapping the system kW, kVA and transient starting events. Given that the line-to-line voltage and current are balanced, use kVA = √3 × V_LL × I_L ÷ 1000 to calculate the apparent power. That means you'll have the alternator power to handle your needs. Keep in mind that this formula is the path to kVA when you have voltage and current and isn't an additional √3 to apply on top of a total kW that you already know.

How Much Extra Capacity Should I Add for Future Growth?

There is no set percentage, and no 20% or 25% to aim for. Determine your growth allowance from a quantified expansion plan with a clear understanding of what's still unknown and how you will back it up. Over-sizing a diesel generator to allow for future expansion which you never reach can result in actual engine damage due to underloading.

Can a Generator Run Below 30% Load?

This depends on the type of engine, instructions for use, aftertreatment system, and maintenance practices. Not all models can be used with one percentage. In the case of a diesel engine, if the engine will be operated at low load for long periods of time, there will be some wet stacking and incomplete combustion, so be sure to check the manufacturer's minimum load guidance for the actual engine before making a decision.

Do VFDs Eliminate Motor-Starting Sizing Problems?

No. Variable frequency drives significantly minimize inrush current, and in many applications, it's no longer the machine that determines the size of the alternator. However, the input harmonics of the drive and the control parameters when accelerating, and the type of load being driven by the generator, must be checked. A VFD doesn't eliminate the sizing problem, it merely alters its form.

Conclusion

Generator sizing is a science, not a game! The machine you really need is determined by your running kW, system power factor, aggressive motor starting surges and the harsh environmental realities.

If you don't know, collect your load schedule and site conditions and send them to us and we will confirm a specific model against them.

Sources

1. ISO, ISO 8528-1:2018 — Reciprocating internal combustion engine driven alternating current generating sets, Part 1: Application, ratings and performance.
2. Schneider Electric, Electrical Installation Guide, Installed power (kW).
3. NIST, Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically.
4. Schneider Electric, Electrical Installation Guide, Induction motors.
5. Schneider Electric, Electrical Installation Guide, Definition of reactive power.
6. Cummins, Specifying and Validating Motor Starting Capability.
7. Caterpillar, A Holistic View on Generator Set Ratings.
8. Cummins Sales and Service, Load Bank Performance Testing.