Diesel Generator Sizing: How to Calculate Power Requirements

Diesel generator sizing means calculating the electrical capacity required to operate connected equipment reliably under normal and peak conditions. The calculation must account for simultaneous running loads, motor starting current, power factor, operating duty, and site conditions. Selecting a generator based only on the total rated power of connected equipment can result in voltage instability, overload trips, or unnecessary equipment costs.

For industrial facilities, the largest motor starting event can determine the required generator capacity even when the normal running load is relatively modest. A reliable selection therefore requires both a load calculation and an assessment of the generator's transient performance.

1. Start with the Actual Electrical Load Profile

The first step is to identify which equipment needs electricity and how it operates. A generator should be sized for the loads that can run simultaneously, not automatically for every machine installed at the facility.

Typical loads include:

• Motors: Pumps, compressors, fans, conveyors, and production machinery.
• Heating equipment: Electric heaters, boilers, and industrial heating systems.
• Lighting: Factory lighting, outdoor lighting, and emergency lighting.
• Electronic equipment: PLCs, computers, control panels, and communication systems.
• Auxiliary systems: Cooling pumps, ventilation, battery chargers, and other supporting equipment.

Prepare a load schedule containing the following information:

In this example, the connected running load is 127 kW if all listed equipment operates simultaneously.

However, this figure does not yet determine the generator size. The two pumps, compressor, and conveyors may have different starting characteristics.

For a factory, obtain equipment nameplates and operating schedules rather than relying entirely on estimated power consumption. Where available, electrical measurements from a power analyzer provide a better picture of actual demand.

2. Understand kW, kVA, and Power Factor

Generators are commonly rated in kilowatts (kW) and kilovolt-amperes (kVA). These measurements describe different aspects of electrical power.

• kW: Real power used to perform useful work.
• kVA: Apparent power supplied by the generator.
• Power factor (PF): The ratio of real power to apparent power.

The basic relationship is:

kVA = kW ÷ Power Factor

For example, a facility requiring 160 kW at a power factor of 0.8 needs:

160 ÷ 0.8 = 200 kVA

This is the apparent power requirement under the stated running conditions. It does not yet account for motor starting or site derating.

For a balanced three-phase system, electrical real power can also be calculated using:

kW = √3 × Voltage × Current × Power Factor ÷ 1,000

The voltage and current must use compatible three-phase line values.

A common mistake is to compare generator ratings using kW alone. An alternator must also accommodate the required kVA, current, and power factor.

3. Why Motor Starting Current Changes Generator Size

Motor starting is one of the most important considerations in diesel generator sizing. Induction motors can draw several times their rated running current during startup, creating a temporary electrical demand that the generator must accommodate.

The starting method influences this demand.

Actual starting current must be taken from motor documentation wherever possible. Typical multipliers are only preliminary estimates.

Consider a factory operating several pumps and compressors. If the largest pump starts while all other equipment is already running, the generator must support the existing load and the pump's starting event.

The selection must also consider:

• Starting kVA of the largest motor.
• Starting power factor.
• Motor acceleration time and mechanical load.
• Permissible voltage dip.
• Engine governor and alternator response.
• Whether other motors can be started sequentially.

Starting motors one at a time can reduce peak demand. This may allow a smaller generator than a design that assumes every motor starts simultaneously.

4. How to Calculate the Required Generator Capacity

A practical generator sizing calculation follows several steps.

1. Calculate simultaneous running demand. Add the real power requirements of equipment expected to operate together.
2. Convert the running load into kVA. Divide the total running kW by the applicable power factor.
3. Identify the largest starting event. Obtain motor starting current or locked-rotor kVA and identify which other loads remain connected during startup.
4. Check transient performance. Compare the starting requirement with the generator manufacturer's motor-starting capability and permissible voltage and frequency deviations.
5. Account for site conditions. Apply manufacturer-specific derating for elevation, ambient temperature, and other relevant installation conditions.
6. Review future demand. Include realistic expansion requirements without adding an arbitrary oversized margin.

Worked Example: Industrial Generator Sizing

Assume a facility has the following requirements:

• Simultaneous running load: 160 kW
• Running power factor: 0.8
• Largest motor: 30 kW
• Starting method: Direct-on-line
• Expected future expansion: Additional production equipment

The running apparent power is:

160 ÷ 0.8 = 200 kVA

This establishes the running-load requirement.

The 30 kW motor must then be assessed separately using its nameplate starting current or locked-rotor data. A simplified assumption of six times rated current would indicate substantial starting demand, but it does not directly establish the final generator rating.

The engineer must determine whether the proposed alternator can support that motor start while maintaining acceptable voltage. The engine must also provide sufficient real power during acceleration.

Therefore, the final generator capacity cannot be established accurately from the 200 kVA running-load calculation alone.

This distinction is important: adding running kVA and starting kVA without considering their different power factors and transient behavior can produce an incorrect result.

5. How Site Conditions and Operating Duty Affect Generator Selection

A generator's nameplate capacity is based on specified reference conditions. Its available output may change when installed in a hot, elevated, dusty, or poorly ventilated environment.

Standby generators are designed for emergency operation under specified conditions. Prime-rated units support defined variable-load operating profiles, while continuous ratings address sustained-load applications under their own conditions.

These ratings should not be treated as interchangeable. Confirm the exact rating definition, permitted operating hours, load limitations, and maintenance requirements for the proposed model.

For example, a factory using a generator as its main power source needs a different operating assessment from a warehouse using it only during occasional grid outages.

6. Common Diesel Generator Sizing Mistakes

Several mistakes can create unnecessary costs or unreliable power supply.

• Sizing only from connected kW: This ignores simultaneous operation, power factor, and motor starting.
• Ignoring the largest motor: A pump or compressor startup may create the most demanding electrical event.
• Applying an arbitrary safety margin: Extra capacity should address identified load growth, transient demand, or site derating rather than replace proper calculations.
• Oversizing excessively: A generator operating continuously at an unsuitable low load may experience poor fuel economics and engine-related operating problems.
• Ignoring non-linear loads: VFDs, UPS systems, and electronic equipment can introduce harmonics that affect alternator selection.
• Confusing standby and prime ratings: A standby rating should not be used as the expected continuous operating capacity.

For additional project planning, ZLN Power's 100 kW diesel generator selection guide discusses capacity considerations for industrial applications.

7. What Information Should You Provide When Selecting a Generator?

A supplier can make a more accurate recommendation when the technical requirements are clearly documented.

Prepare the following information:

• Total simultaneous running load in kW.
• Required voltage, frequency, and phase arrangement.
• Power factor of the connected equipment.
• Largest motor rating and starting method.
• Motor starting sequence and acceptable voltage dip.
• Required prime, standby, or continuous duty.
• Site elevation and maximum ambient temperature.
• Enclosure, noise, and installation requirements.
• Expected future load growth.

When comparing ZLN Power diesel generator solutions, provide this information alongside the intended application. Request the proposed model's rating, fuel-consumption curves, motor-starting capability, and site derating data.

The supplier's recommendation should be checked against the actual equipment list and the generator manufacturer's technical specifications.

Conclusion

Correct diesel generator sizing requires more than adding the rated power of connected equipment. Running demand, motor starting, power factor, transient response, site conditions, and operating duty must all be considered.

The most reliable approach is to prepare a detailed load schedule, identify the largest starting event, and verify the proposed generator against manufacturer performance data. This produces a more practical selection and helps avoid the cost of both undersized and unnecessarily oversized equipment.

Frequently Asked Questions

How do I calculate the right diesel generator size?

Add the equipment expected to operate simultaneously, convert the running load from kW to kVA using the power factor, and assess the largest motor-starting event. Then check site derating and the manufacturer's transient performance data.

What is the difference between kW and kVA in generator sizing?

kW measures real electrical power, while kVA measures apparent power. Their relationship depends on power factor: kVA equals kW divided by power factor.

How much extra capacity should a diesel generator have?

There is no universal reserve percentage suitable for every project. The required allowance depends on future load growth, motor starting, site derating, and the generator's operating duty.

Does a motor require a larger generator when starting?

Often, yes. Motor starting current can be several times higher than running current. The required generator capacity depends on the starting method, motor characteristics, existing load, and acceptable voltage dip.

Can an oversized diesel generator cause problems?

Yes. Excessive oversizing can increase purchase and operating costs. Prolonged low-load operation may also create engine maintenance concerns, so the generator should be matched to its expected load profile.

Reference Sources

• ISO 8528-1:2018 – Generator-set application, ratings, and performance classifications.
• ZLN Power – 100 kW Industrial Diesel Generator Selection Guide.

Technical note: Generator sizing examples are for preliminary planning. Final selection should be verified using actual equipment data, manufacturer performance curves, and site-specific engineering calculations.