In short. A diesel generator is sized in six steps: list the loads that run together, convert to kVA at the real power factor, find the hardest transient such as the largest motor start or UPS block load, choose the rating category for the duty, derate for ambient temperature and altitude, and check that normal running load sits between roughly 30 and 80 percent of the rating. The largest step load, not the total, usually decides the final kVA.
Why the total kW is the wrong place to stop
Most sizing errors come from treating the generator as a bucket that only needs to be bigger than the sum of the loads. A generator is a rotating machine with an engine that must produce torque within a fraction of a second, an alternator whose voltage sags when current surges, and a controller that shuts it down if either excursion goes too far. Two sites with identical running loads of 300 kW can need very different sets if one starts a 90 kW direct-on-line compressor while the other runs only lighting and servers through a UPS. Sizing means finding the worst case the set will face and selecting a package that survives it with margin, without idling at low load for most of its life.
The six-step sizing method
1. Establish the simultaneous running load
Begin with a load inventory that records each load’s kW, power factor, starting method and whether it operates during an outage. Separate connected load from the demand that actually occurs together, and use the recorded maximum demand on the DISCOM bill as a cross-check.
2. Convert to apparent power
kVA = kW / PF
Here kW is the real power of the simultaneous load and PF is its power factor, measured where possible. Generators are rated in kVA at 0.8 PF lagging, so the engine behind a set delivers about kVA × 0.8 in kW. A 240 kW load at 0.8 PF draws 300 kVA. The same 240 kW at a measured 0.92 PF draws only 261 kVA, which lowers current, cable loading and alternator heating, but the engine still has to supply 240 kW, so a set rated at 0.8 PF still needs at least 240 ÷ 0.8 = 300 kVA. Take the larger of the load kVA and the load kW ÷ 0.8, then apply the duty, site and transient checks below. A higher load power factor can allow a smaller alternator frame on a separately matched engine; it never removes the engine kW limit. The kW to kVA guide covers the detail.
3. Identify the hardest transient
Find the single event that puts the greatest instantaneous demand on the set: the largest motor started direct-on-line, a group of motors restarting together after transfer, transformer inrush, a UPS taking load while recharging batteries, or a block of load applied in one step by an ATS. Work out the starting kVA of that event and the load already on the set when it occurs. The transient then has to be checked against the alternator’s motor-starting capability at an acceptable voltage dip and against the engine’s load acceptance.
4. Choose the rating category
Manufacturers publish ratings in the categories defined by ISO 8528-1, and using the wrong one is a common cause of early engine wear.
| Rating | Intended duty | Typical use in India |
|---|---|---|
| Emergency standby (ESP) | Variable load during utility outage, limited hours per year, no overload | Offices, apartments, hospitals and factories with reliable DISCOM supply |
| Prime (PRP) | Unlimited hours at variable load, average about 70 percent of rating, 10 percent overload for short periods | Sites with long daily outages, construction, rental fleets |
| Limited-time running power (LTP) | Constant load for a limited number of hours per year | Peak-lopping or scheduled load support |
| Continuous (COP) | Constant load, unlimited hours, no overload | Captive power plants, remote sites with no grid |
A standby-rated set pressed into eight hours of daily running during a long shortage is operating outside its rating even if it never trips.
5. Derate for the site
Ratings assume reference conditions, typically 25 degrees Celsius intake air and up to 100 metres altitude. A canopy set in a Hyderabad yard in May breathes air at 45 degrees Celsius or more, engine output falls with air density, and the alternator reaches its temperature rise limit sooner. Exact derating percentages must come from the engine data sheet, but allow several percent for high ambient and confirm the 500 to 600 metre altitude of the Deccan plateau against the engine’s allowance.
6. Check the operating band
After adding margin for the transient and for planned growth, confirm that typical running load still sits comfortably within the rating. Turbocharged engines, and CPCB IV+ engines with aftertreatment in particular, do not tolerate long periods below about 30 percent load: unburnt fuel reaches the exhaust, the turbocharger stays cold and the aftertreatment demands regeneration. If the margins push normal load down to 20 percent, consider two smaller sets, a soft starter on the problem motor or staged load application instead.
A worked example (illustrative)
A packaging plant outside Hyderabad has a measured maximum demand of 210 kW at 0.88 PF during the shift that must be protected, which is 239 kVA. The largest motor is a 45 kW air compressor started direct-on-line with a locked-rotor current of about six times full-load current, a starting demand in the region of 350 kVA at low power factor. The compressor cycles while the plant is running, so the start happens with roughly 190 kVA already on the set.
Start with the engine. 210 kW on a set rated at 0.8 PF needs at least 210 ÷ 0.8 = 262.5 kVA, even though the load itself draws only 239 kVA. A 250 kVA set (200 kW) is therefore too small on kW alone, whatever the power factor. The next standard rating, 320 kVA (256 kW), carries the running load at about 82 percent of its kW rating before any growth margin or site derating.
The motor start then decides between two options. If the compressor stays direct-on-line, a 350 kVA start on top of about 190 kVA running produces, on typical alternator data, a voltage dip that contactors and drives will not tolerate on a 320 kVA set, so a 380 to 400 kVA set (or a 320 kVA set with an oversized alternator) is needed. Alternatively, a soft starter limiting the compressor to about three times full-load current keeps the 320 kVA set. The soft-starter option usually costs less in total and avoids the light-load problems a 400 kVA set would face overnight. These figures are illustrative lower bounds before growth margin and derating; confirm the shortlisted set with the manufacturer’s sizing output, because motor-starting capability differs between alternators of equal rating.
Margins that are reasonable and margins that are not
A margin of 10 to 25 percent above the calculated kVA for standby duty is normal practice. A defined future load, listed with its own starting characteristics, is good practice; a vague doubling “for the future” is not. Oversizing the alternator alone, one or two frames above the engine, improves motor starting and harmonic performance without raising the engine’s minimum load. Taking the next frame size “to be safe” after every step compounds into a set far larger than needed that runs lightly loaded for years.
Single set or multiple sets
Above about 500 kVA, or wherever load varies widely between day and night, compare one large set with two or three paralleled sets. Multiple sets let one machine carry the night load efficiently and provide N+1 redundancy for maintenance, at the cost of a synchronising panel and floor space. See integrated power systems.
Related photographs and videos
Common mistakes
- Adding every nameplate kW and applying no diversity, which selects a set that runs at a fraction of its rating and wet-stacks within months.
- Assuming 0.8 power factor for every load, which oversizes plants with mostly resistive or corrected loads and undersizes plants with many lightly loaded motors.
- Skipping the largest motor start, so the set carries the running load in the showroom and stalls on the first compressor restart on site.
- Forgetting ambient derating, so a set that was adequate at commissioning in December trips on overload the following May.
What to send us
Send your load list with starting methods for motors above about 10 kW, three months of electricity bills showing maximum demand, details of any UPS or variable-speed drives, the expected hours of running per day, and whether the set will be indoors or in a canopy. If you are replacing an existing set, add a photograph of its nameplate and a note of any problems it has had. Send a generator sales enquiry and we will return a sizing recommendation with the assumptions stated.
Sources and assumptions
- Rating categories (ESP, PRP, LTP, COP): ISO 8528-1, Reciprocating internal combustion engine driven alternating current generating sets — Application, ratings and performance.
- Load acceptance and transient performance classes (G1 to G4): ISO 8528-5, Generating sets.
- Sizing method, motor starting and voltage dip: generator manufacturers’ application manuals, for example Cummins Power Generation T-030, Liquid-cooled generator sets application manual.
- Assumptions used in the examples: generator rated at 0.8 PF lagging, 415 V 50 Hz, site conditions within the manufacturer’s reference rating (no altitude or temperature derating), typical alternator transient reactance. The worked examples are illustrative lower bounds, not a selection; the final set is confirmed against the manufacturer’s sizing output and data sheet for the specific model.
Questions engineers ask
What percentage margin should I add to the calculated load?
For standby duty, 10 to 25 percent above the simultaneous running kVA is typical, provided the largest starting event has been checked separately. The transient check often sets the size, in which case no further margin is needed for running load.
Is it better to oversize a generator to be safe?
Only up to a point. A set running below about 30 percent load for long periods suffers carbon build-up, oil contamination and, on CPCB IV+ engines, aftertreatment problems. Handle transients with soft starters, staged loading or multiple sets rather than raw kVA.
Can I size from the electricity bill alone?
The recorded maximum demand is a useful anchor for running load because it already includes diversity, but it says nothing about starting events or which loads must stay on during an outage, so pair it with a load inventory.
What is the difference between standby and prime rating for the same set?
The same engine and alternator carry about 10 percent less load in prime duty than in standby, and prime duty expects the average load over 24 hours to stay around 70 percent of the prime rating. Standby ratings assume limited hours per year during utility failure.
References and provenance
The standards and manufacturer documents named on this page, with the edition used (obtainable from ISO, BIS or the manufacturer):
- ISO 8528-1:2018, Reciprocating internal combustion engine driven alternating current generating sets, Part 1: Application, ratings and performance (defines the rating categories ESP, PRP, LTP and COP; current edition 2018).
- ISO 8528-5:2025, Part 5: Generating sets (performance classes G1 to G4, load acceptance and transient voltage and frequency limits; current edition 2025, which replaced the 2022 edition).
- Cummins Power Generation, Application Manual T-030, Liquid Cooled Generator Sets, document A040S369, Issue 9 (February 2015): chapter 3, Load impacts (including motor starting and transient voltage and frequency effects), and chapter 4, Equipment selection (generator set and alternator sizing). Available from Cummins and its dealers; not linked here.
Worked example status. The 320 kVA versus 380 to 400 kVA comparison for the direct-on-line compressor start is an illustration using typical alternator transient reactance (about 0.2 per unit) and the dip formula shown on this page. It is not a manufacturer sizing output. For a real project the selection is confirmed with the manufacturer’s sizing software or alternator motor-starting curves for the specific model, and that printout is attached to the quotation.
Looking for a specific rating? See the generator full-load current chart for every common size from 7.5 to 3,000 kVA at 415 V and 11 kV.