Diesel Generator Load Calculation: Running & Starting

A diesel generator load calculation answers two separate questions. First, how much power the site consumes while everything is running. Second, how much surge the set must absorb for the few seconds it takes the largest motor to start. On industrial sites the second number decides the frame size as often as the first. The method below builds a load list in running kW, converts it to running kVA, and then tests the worst-case starting condition against what the alternator can actually deliver.

The Three Loads You Have to Keep Separate

Most sizing errors come from treating "load" as a single number. It is three numbers, and each one stresses a different part of the machine.

Running load

This is the real power, in kW, that all connected equipment draws in steady state. It is what the engine has to produce continuously, and it drives fuel consumption, cooling system sizing and the prime power rating.

Apparent load

This is the kVA the alternator must supply. It is larger than the kW figure whenever the load power factor is below 1.0, because part of the current does no work at the shaft. Alternators are rated in kVA for exactly this reason. Our breakdown of kVA vs kW in generator ratings explains the relationship in detail.

Starting load

An induction motor at standstill behaves like a transformer with a shorted secondary. For a few hundred milliseconds it draws a large inrush current at a very low power factor. That surge is a kVA problem for the alternator and a voltage dip problem for every other load on the bus.

Building the Load List: What to Record and Which Multipliers Apply

Work from a list, not from a total. For every significant load, record four things before you calculate anything:

• The nameplate rating in kW or hp, and the rated voltage and number of phases
• The power factor and, for motors, the starting method
• The duty cycle — continuous, intermittent, or standby-only
• The order in which loads are switched on after a power failure

The last item is the one most lists omit. If two large motors can start at the same instant, the set must be sized for that combined surge. If a control system enforces sequential starting, the required capacity drops sharply.

The inrush multiple depends on how the motor is started, not only on the motor itself. The table gives typical planning ranges in multiples of full-load current.

Treat these as planning ranges only. The governing figure is the locked-rotor current on the motor nameplate or the NEMA code letter, which gives locked-rotor kVA per horsepower. For a step-by-step treatment of that calculation, see our guide to choosing the right diesel generator size.

Running kVA, Worst-Case Surge and Voltage Dip

Convert each load with kVA = kW ÷ power factor, then evaluate the worst starting case using this expression:

Worst-case starting kVA = total running kVA − largest motor running kVA + largest motor starting kVA

Worked example for a small factory:

The set must therefore carry 140 kW at an overall power factor of about 0.89, which is roughly 158 kVA continuously. The worst case occurs if the pump starts while everything else is already running: 158 − 35 + 212 = 335 kVA.

Note what happens to the engine during that start. Motor inrush sits at a power factor around 0.2–0.4, so 212 kVA of surge is only about 60–85 kW of real power. The engine therefore sees roughly 175–195 kW for a second or two, against a steady running load of 140 kW.

Carrying the surge is only half the test. The alternator's voltage will fall during inrush and must recover quickly enough that contactors do not drop out and drives do not trip. Many industrial specifications target a transient voltage dip below roughly 15–20% with recovery within a few seconds, but the number is a project decision, not a universal constant.

The dip is determined by the alternator's sub-transient reactance and the excitation system's response. A machine with permanent magnet generator (PMG) excitation and a fast AVR recovers better than a self-excited machine of the same kVA. Ask for transient data for the specific alternator frame rather than relying on the kVA number alone.

Diversity, Minimum Load and Other Corrections

Diversity accounts for the fact that not every load runs at once. On mixed industrial sites the applied factor is often 0.7–0.9, but it must be measured or agreed with the plant owner, not invented.

Then check the opposite failure mode. Engines that run for long periods at very light load accumulate unburned fuel and soot in the exhaust tract, a condition known as wet stacking. Many manufacturers advise keeping sustained loading above roughly 30–40% of rated output.

Two further corrections are worth applying. Switch-mode power supplies, UPS rectifiers, variable frequency drives, LED drivers and battery chargers draw current in pulses rather than a sine wave. The harmonic content heats the alternator winding and distorts the voltage waveform, which affects other equipment on the same bus.

Where non-linear loads are a large share of the total, allow for them explicitly. Common mitigations are an oversized alternator frame, a two-thirds pitch winding, PMG excitation, and an AVR with true three-phase sensing.

Do not confuse motor inrush with step loading. Step loading is a block of load applied at once — a building transferring onto standby power, or a bank of heaters switching in. Here the limit is usually the engine and turbocharger response rather than the alternator. Large block loads are better applied in stages by the control system than absorbed in one step.

Mistakes That Produce an Undersized Set

• Sizing on nameplate kW only and ignoring the starting surge
• Assuming a power factor of 1.0 for the whole site
• Ignoring the starting sequence and assuming simultaneous starts
• Overlooking derating for altitude and ambient temperature
• Ignoring single-phase loads and phase unbalance on a three-phase set
• Ignoring harmonic content from UPS and drive loads
• Sizing for today's load with no allowance for expansion

The derating point matters. A set rated at sea level and 25 °C will not deliver the same output at 2,000 m in high ambient heat. Our diesel generator specification guide covers the parameters that change with site conditions.

What to Send Your Supplier Before Requesting a Quote

Send a load list with starting methods and switching sequence, the allowable voltage dip, site altitude and temperature range, fuel specification, the duty classification you need, and the split between single-phase and three-phase load. If the running profile is not fixed yet, settle the distinction between prime and standby power ratings first, because it changes the rating you compare against.

A diesel generator load calculation is only as good as the list behind it. You can review frame sizes across our diesel generator set range to see how ratings are structured before finalising it.

FAQ

Should I size by kW or by kVA?

Both, for different reasons. The engine is limited by kW and the alternator by kVA. Calculate the running kW for the engine rating, then confirm the alternator kVA covers both the running apparent load and the worst-case starting surge.

How much margin should I leave above the calculated load?

Enough to cover known expansion while keeping the normal running point in a healthy part of the load range, rather than as large a margin as the budget allows.

Can a soft starter really reduce the generator size?

It usually reduces the required starting kVA substantially, and therefore the voltage dip. Whether it changes the frame size depends on how much of your total requirement is set by the surge rather than the running load.

What if I cannot get locked-rotor data for the motors?

Use the NEMA code letter on the nameplate, which gives locked-rotor kVA per horsepower, or ask the motor supplier for locked-rotor current.

Reference Sources

• ISO 8528-1:2018 — Application, ratings and performance of generating sets
• ISO 8528-5:2025 — Generating sets, design and performance criteria
• NFPA 110 — Standard for Emergency and Standby Power Systems
• IEC 60034-1:2022 — Rotating electrical machines, rating and performance
• EGSA — Electrical Generating Systems Association