Sizing a Diesel Generator: Loads, Starting Duty and Operating Conditions

The quickest way to size a generator is to add up the nameplate ratings of everything it might supply. It is also one of the least reliable. A nameplate sum mixes kW and kVA, counts standby equipment that never runs, and ignores diversity, so it often overstates the running load. It also says nothing about the moment a large motor starts, when the generator may briefly have to supply several times that motor’s running kVA at a poor power factor. A set sized from nameplates can therefore be too large for its everyday load and still too weak for its starting duty.

This article answers the question: how is a diesel generator sized from the actual load, which quantities decide the size, and which checks must be confirmed with the manufacturer before a set is selected?

Why nameplate summation goes wrong

A typical first attempt lists every connected item and adds the ratings. This goes wrong in several ways:

• Mixed units. Motor nameplates give shaft output in kW. UPS and transformer nameplates give kVA. Lighting is listed in watts. Adding them produces a number with no physical meaning.
• Equipment that never runs together. Duty and standby pumps, redundant fans, and seasonal heating and cooling loads are all counted, although only some will run at once.
• Ratings are not demand. A UPS rated 80 kVA may supply 40 kW of IT load, and a motor rated 22 kW may run at 70% load.
• Starting is missing. The largest transient demand, a motor starting while other loads are running, does not appear on any nameplate.

Sizing from the actual load corrects all of these.

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Step 1: Define the duty and the rating category

Before calculating, decide how the generator will be used. ISO 8528-1 defines power rating categories that link the set’s declared power to its duty:

Category	ISO 8528-1 concept (summarised)	Typical application
ESP (emergency standby power)	Maximum power available during a variable load sequence, for up to 200 h of operation per year, in the event of a utility outage or under test conditions. The permissible average power over 24 h is not to exceed 70% of ESP unless otherwise agreed with the engine manufacturer	Standby sets for infrequent mains failures
PRP (prime power)	Maximum power available with a variable load for an unlimited number of hours per year. The permissible average power over 24 h is not to exceed 70% of PRP unless otherwise agreed	Sites where the generator is the main source, or outages are long or frequent
LTP (limited-time running power)	Maximum power available for up to 500 h per year	Planned limited running, such as peak lopping, where agreed
COP (continuous power)	Maximum power available continuously at a constant load for an unlimited number of hours per year	Base-load and continuous operation
DCP (data centre power)	Maximum power available with a variable or continuous load for unlimited hours in data centre applications. The manufacturer defines the power level; prolonged parallel operation with the utility is not permitted	Data centres with a reliable utility supply

Three practical points follow:

• The same set has different ratings for different categories. A set’s ESP rating is higher than its PRP rating. Comparing offers where one quotes ESP and another PRP compares different duties.
• The average-load limit can govern. For ESP and PRP, the expected average load over 24 h must stay within the permitted proportion of the rating. During a long outage this can require a larger set than the peak load alone suggests.
• Manufacturers’ own rating names differ. Many publish additional or differently named ratings. Ask which ISO 8528-1 category each quoted figure corresponds to.

Generator ratings are explained in more depth in a separate Engineering Insights article. The roles of the generator alongside the UPS and ATS are covered in How Generators, UPS and ATS Work Together.

Step 2: Build the load schedule

List every load the generator will supply, with:

• Real running power in kW, at the input terminals. For motors, this is shaft power divided by efficiency, at the expected load, not the nameplate output.
• Power factor at running load, giving kVAr.
• Load type: linear (heaters, most lighting), motor (with starting method: direct-on-line, star-delta, soft starter, variable speed drive), or non-linear (UPS rectifiers, VSDs, LED drivers, IT power supplies).
• Demand or utilisation factor: the proportion of rated demand actually drawn, from measurements, process data or design assumptions that are documented.
• Operating group: which loads run together, which are duty/standby pairs, and which are interlocked so they cannot run at the same time.
• Priority and step: whether the load is connected in the first step, a later step, or not at all on generator supply.

The schedule is the single most important sizing document. Every number in it should have a source: a measured value, a datasheet, or a stated assumption.

Step 3: Running demand: kW, kVAr, kVA and power factor

For each load, calculate:

• P (kW) = running input power × demand factor
• Q (kVAr) = P × tan(arccos(pf))

Then add P and Q separately across all loads that run together, and find the following. This is the same demand-based logic used to size transformers in Sizing a Distribution Transformer:

• S (kVA) = √(P² + Q²)
• Overall power factor = P / S

Why this matters:

• The engine must deliver the total kW. The alternator must deliver the total kVA at the resulting power factor.
• Generator sets are commonly rated at a lagging power factor of 0.8. If the load’s power factor is higher than the set’s rating basis, the engine kW limit is usually reached first. If it is lower, the alternator kVA limit may be reached first. Check both against the manufacturer’s rating basis.
• Leading (capacitive) loads, such as lightly loaded UPS input filters or power factor correction capacitors left connected, can disturb the alternator’s voltage control. Find out from the alternator manufacturer how much leading kVAr it can accept.

Step 4: Motor starting: the duty that often decides

When an induction motor is started direct-on-line (DOL), it draws its locked-rotor current, typically several times its full-load current, at a low power factor until it accelerates. The motor manufacturer’s data gives the exact ratio. In kVA terms:

Starting kVA ≈ √3 × U × I_start

where I_start is the locked-rotor current at rated voltage.

On a generator, this sudden demand causes:

• a voltage dip, set mainly by the alternator’s reactances and the response of its excitation and voltage regulator;
• a frequency dip, because the engine must respond to the sudden real-power step (the starting kW is lower than the kVA, but not zero).

If the dip is too deep, the motor may fail to accelerate, contactors may drop out, other loads may trip, and the UPS may reject the generator supply.

ISO 8528-5 defines the transient performance of a generating set through performance classes (G1 to G4), which set permitted limits for voltage and frequency deviation and recovery time on load changes. The required class depends on how sensitive the connected loads are. Whether a particular set meets the required class for a particular motor start is determined from the manufacturer’s data: motor-starting curves, which relate starting kVA to voltage dip, and transient-response data or sizing software. The figures cannot be derived reliably from a generic formula.

How the starting method changes the duty:

Starting method	Effect on the generator (qualitative)
Direct-on-line	Highest starting kVA, low power factor. Usually the worst case
Star-delta	Starting current reduced to roughly a third of DOL, with a second transient at changeover. Starting torque also reduced
Soft starter	Starting current limited to a set value. Lower torque. Some harmonic distortion during the start
Variable speed drive	Starting current close to running current and ramped. Continuous harmonic current must be considered

Step 5: Load steps and sequencing

When the generator takes over from the mains, the ATS or controller connects the loads. Connecting everything at once creates one large step, often more than the engine can accept without an excessive frequency dip. Turbocharged diesel engines in particular have a limited ability to accept large sudden loads, and manufacturers state the load-acceptance capability of each set. ISO 8528-5 provides the framework for load acceptance and transient performance.

Good practice:

• Divide the loads into steps, each within the set’s declared load-acceptance capability, connected a few seconds apart once voltage and frequency have recovered.
• Put life-safety and critical loads in the first step.
• Give large DOL motors their own step. Starting them early, while the base load is small, often reduces the total transient. However, a motor started late, with other loads already connected, experiences a dip of a similar size and also affects everything else that is running.
• Check the worst credible case, not only the planned sequence. Pumps on pressure switches, compressors and HVAC units restart automatically and may start while everything else is already running.
• Account for UPS walk-in: the UPS rectifier should ramp its input gradually rather than appear as a block step.

Step 6: Non-linear loads

UPS rectifiers, VSDs and electronic power supplies draw harmonic currents. On a generator, which has a much higher source impedance than a transformer-fed mains supply, these currents cause:

• voltage distortion, which can disturb other loads and controllers;
• additional heating in the alternator;
• in some cases, interaction with the voltage regulator.

The usual response is to select an alternator with sufficient margin for the non-linear load, sometimes a larger alternator on the same engine. The manufacturer evaluates this using the harmonic spectrum of the load, from the UPS or drive data. Input filters and active front ends reduce the problem. They should be specified with the generator in mind.

Step 7: Operating and site conditions

Generating set ratings are declared at standard reference conditions. ISO 8528-1 uses a total barometric pressure of 100 kPa, an air temperature of 25 °C and a relative humidity of 30%, consistent with ISO 3046-1 for the engine. Where the site differs, the available power changes:

• Altitude. Lower air density reduces engine power and alternator cooling.
• High ambient temperature. Reduces engine power and alternator capacity, and must be within the radiator’s cooling capability. The relevant temperature is the air entering the set and its radiator, which in an enclosure or plant room can be well above outdoor ambient.
• Humidity, fuel quality and air quality. Dust, salt and other contaminants affect filters, cooling and durability.

Derating is manufacturer-specific. Each engine and alternator has its own derating data, depending on turbocharging, aftercooling, cooling system design and enclosure. Generic tables or rules of thumb cannot replace them. Specify the site conditions and ask the manufacturer to declare the site-rated power for the rating category required.

Step 8: Future capacity, and the risk of oversizing

• Include identified future loads: planned extensions, equipment already budgeted, or a capacity margin the owner has deliberately set. Record the reason for each.
• Do not apply a universal oversizing percentage. It hides the real assumptions and can lead to the opposite problem.
• Light loading is also a concern. Diesel engines run for long periods well below their rating can suffer from incomplete combustion and fuel and lubricating-oil deposits in the exhaust system, often called “wet stacking”. Manufacturers state recommended minimum loading, and periodic testing at load is often specified to avoid it.
• Where both future capacity and light-load operation matter, multiple paralleled sets can be a better answer than one large set (see How Generators, UPS and ATS Work Together).

Step 9: Manufacturer verification

The calculation gives the requirement: site kW, site kVA, power factor, the load-step plan and the worst-case starting duty. The manufacturer confirms that a specific set meets it, using:

• the set’s ratings in the required ISO 8528-1 category at the stated site conditions;
• alternator data (reactances, motor-starting curves, capability chart, harmonic capability);
• transient response and load-acceptance data against the required ISO 8528-5 performance class;
• sizing software results for the planned load steps and the worst-case motor start.

Illustrative worked example

Scenario (illustrative, not a design): a standby generator for a small facility at 400 V, 50 Hz, three-phase. The figures are invented to demonstrate the method and do not describe any real product or site.

Assumptions

#	Assumption	Source in a real project
A1	Duty: standby for infrequent mains failures, well under 200 h/year; ESP category considered	Owner’s outage history and requirements
A2	Lighting and small power: 40 kW connected, pf 0.95, demand factor 0.9	Lighting schedule, measured data
A3	Mixed HVAC and ancillary loads (small motors, linear): 60 kW connected, pf 0.85, demand factor 0.8	HVAC design, measured data
A4	UPS rated 80 kVA, supplying 40 kW of critical load. Input demand including battery recharge: 50 kW at pf 0.95	UPS manufacturer’s input data
A5	Duty pump: 55 kW motor, efficiency 0.94, pf 0.86, direct-on-line, locked-rotor current 7 × full-load current, starting pf 0.30	Motor datasheet
A6	Standby pump: 55 kW, interlocked with the duty pump, never runs at the same time	Control philosophy
A7	Fan: 22 kW motor on a VSD. Input ≈ 22 / 0.93 (motor efficiency) / 0.97 (drive efficiency) ≈ 24.4 kW, displacement pf 0.95	Motor and drive data
A8	Future load identified by the owner: one additional 30 kW load, pf 0.85	Owner’s development plan
A9	Estimated 24-hour average load during an extended outage, including the future load: 175 kW	Operating profile
A10	Site: 1 500 m altitude, 40 °C air temperature at the set’s intake	Site survey

Nameplate sum for comparison: 40 + 60 + 80 + 55 + 55 + 22 = 312 (a meaningless mix of kW and kVA that includes the standby pump).

Running demand

S is calculated from the summed P and Q (√(246.9² + 119.3²) ≈ 274.2 kVA), not by adding the kVA column.

Average-load check (ESP)

With an estimated 24-hour average of 175 kW (A9), and the ISO 8528-1 limit on average power for ESP of 70% unless otherwise agreed:

Required ESP ≥ 175 / 0.70 = 250 kW

This is slightly above the peak running demand of 246.9 kW. The average-load rule governs the kW requirement here. If outages were expected to be long or frequent, a PRP-rated selection would be considered instead.

Motor starting

Duty pump full-load current:

I_FL = 58 500 W / (√3 × 400 V × 0.86) ≈ 98.2 A

Locked-rotor (DOL starting) current (A5):

I_start = 7 × 98.2 ≈ 687 A

Starting kVA:

S_start = √3 × 400 V × 687 A ≈ 476 kVA, at pf 0.30, so ≈ 143 kW and ≈ 454 kVAr

That is about seven times the pump’s running kVA, and well above the facility’s entire running kVA.

Load steps

Worst credible case: the duty pump is controlled by a pressure switch and can restart while everything else, including the future load, is running. Base load without the pump is ≈ 188.4 kW and 84.6 kVAr. During the restart the generator sees ≈ 331 kW and ≈ 539 kVAr, about 633 kVA, for the duration of the start. The voltage dip in this case, not the running demand, is likely to decide the alternator size.

Site conditions

The site (A10) is above the 100 kPa / 25 °C / 30% RH reference conditions. The manufacturer must declare the site-rated ESP for each candidate set. The selection condition is:

Site-rated ESP (from manufacturer) ≥ 250 kW, and site-rated kVA ≥ 274 kVA at a load pf of about 0.90

This example deliberately applies no generic derating factor. The real factor depends on the specific engine, alternator and cooling package.

Result: what the requirement looks like

Requirement	Value from this example	To be confirmed by manufacturer
Rating category	ESP (A1)	Rating basis of the quoted figure
Site-rated real power	≥ 250 kW (average-load rule governs)	Site-rated ESP at 1 500 m, 40 °C
Site-rated apparent power	≥ 274 kVA at pf ≈ 0.90	Alternator capability at site conditions
Motor starting	≈ 476 kVA at pf 0.30 (DOL pump); worst case ≈ 633 kVA total during restart with all loads running	Voltage and frequency dip against the required ISO 8528-5 class, using motor-starting curves or sizing software
Load steps	Three steps as above	Load acceptance of the engine per step
Non-linear load	UPS ≈ 50 kW, VSD ≈ 24 kW	Alternator margin for harmonic current

Options if the starting check fails: replace DOL with a soft starter or VSD for the pump, which can cut starting kVA substantially. Alternatively, select a larger alternator on the same engine, or interlock the pump restart so that it can only start under controlled conditions. Each option changes cost and operation differently and should be compared, not chosen by default.

Limitations: the example ignores cable voltage drop, unbalance, regenerative loads and any regulatory requirements for safety services, such as fire pumps, which may impose their own rules. It is a requirement statement, not a generator selection. The final selection must be verified with the manufacturer’s data.

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Specification and procurement considerations

An enquiry for a generator set should provide:

• the load schedule with real kW, power factor, load type and starting method, grouped by operating group and load step;
• the duty: expected running hours per year and outage durations, and the ISO 8528-1 rating category required;
• the required ISO 8528-5 performance class, or the permissible voltage and frequency deviation for the connected loads;
• the worst-case motor start and the planned load-step sequence;
• non-linear load details: UPS and VSD ratings, harmonic data if available;
• site conditions: altitude, maximum air temperature at the intake, humidity, and installation (indoor plant room or outdoor enclosure);
• identified future loads;
• interface requirements: voltage, earthing arrangement, ATS or paralleling controls, fuel autonomy, noise limits and emissions requirements of the destination market.

Ask offers to state the site-rated power in the requested category, the alternator’s motor-starting capability, and the basis for both.

AdepaTrade sources diesel generator sets and associated critical power equipment to project specifications from manufacturers in Türkiye and worldwide, and can coordinate manufacturer sizing checks as part of an enquiry. It does not carry out installation. On request, AdepaTrade can arrange on-site engineering supervision and commissioning support under a separate scope.

Common mistakes

• Sizing from the sum of nameplate ratings, mixing kW and kVA and counting standby equipment.
• Adding kVA arithmetically instead of summing kW and kVAr separately.
• Checking only the running load and ignoring the largest motor start, or checking it only in the planned sequence when the motor can restart automatically at any time.
• Sizing for the UPS output rating rather than its input demand including battery recharge.
• Comparing ESP figures from one offer with PRP figures from another, or quoting a manufacturer-specific rating as if it were an ISO 8528-1 category.
• Applying a generic altitude or temperature derating instead of the manufacturer’s data, or using outdoor ambient temperature when the set breathes hot plant-room air.
• Adding a large, unexplained oversizing margin, which raises cost and can cause light-load problems.
• Treating a simplified hand calculation as proof that a set will accept a motor start.

Conclusion

A generator is sized correctly when it meets three requirements at once: enough kW for the real running and average load in the chosen ISO 8528-1 rating category, enough kVA at the actual power factor, and enough transient capability to start the largest motor and accept each load step within the limits the loads can tolerate. Site conditions reduce what the set can deliver relative to its catalogue rating, and only the manufacturer can say by how much. A clear load schedule with documented assumptions turns generator sizing from guesswork into a requirement statement that manufacturers can verify, and that procurement can use to compare offers on the same basis.

Related AdepaTrade solutions

AdepaTrade sources diesel generator sets, UPS systems, batteries and associated transfer and distribution equipment. Critical & Backup Power · Low Voltage Distribution & Control

Relevant standards

IEC standards are the primary reference for the electrical equipment; generating sets themselves are covered by the ISO 8528 series, with ISO 3046-1 for the engine. Confirm the applicable edition for each project.

Standard	What it covers	Why it matters
ISO 8528-1: RIC engine driven AC generating sets, Part 1: Application, ratings and performance	Rating categories ESP, PRP, LTP, COP, DCP; average-load limits; reference conditions; site rating	The starting point for sizing: choose the rating category for the duty, then confirm the output available at site conditions
ISO 8528-5: Part 5: Generating sets	Performance classes G1–G4; transient response; load acceptance	Determines how much load the set can accept in one step and how it recovers; state the class the loads require
ISO 8528-3: Part 3: Alternating current generators for generating sets	Alternator ratings	The alternator, not only the engine, must carry the kVA and the motor-starting demand
ISO 3046-1: RIC engines, Performance, Part 1	Engine standard reference conditions and power adjustment	Site altitude, temperature and humidity can reduce available power; ask for the site rating, not only the rating at reference conditions
IEC 60034-1 / IEC 60034-12: Rotating electrical machines (rating; starting performance of cage induction motors)	Motor ratings and starting characteristics	Motor starting current and power factor drive the largest transient load the generator must supply
ISO 8528-12: Part 12: Emergency power supply to safety services	Additional requirements where the set supplies safety services	Safety-service duty may add requirements beyond those of standby supply