# Diesel Generator Sizing: kW vs kVA, Prime and Standby Ratings > Diesel generator sizing starts with running kW, motor starting load and power factor, then the right ISO 8528 rating. Learn kW vs kVA, prime and standby. Engines & Power By [MARS Engineering & Sales Team](/about/) Published Oct 23, 20267 min read Diesel generator sizing means adding up the running kW of every load, allowing for the extra current motors draw when they start, converting to kVA at the generator's 0.8 power factor, and then choosing a set whose standby or prime rating under ISO 8528-1 covers that demand with a sensible margin. For example, 100 kW of mixed load with a large motor typically needs a set of at least 125 kVA, and often larger once motor starting and a 10–25 percent margin are included. ## Key takeaways - kVA is apparent power and kW is real power; for a three-phase genset, kVA equals kW divided by the power factor, normally 0.8, so 100 kW equals 125 kVA. - Under ISO 8528-1, an emergency standby rating is limited to about 200 hours a year at an average load of no more than 70 percent; a prime rating allows unlimited hours at a variable load averaging no more than 70 percent. - Direct-on-line induction motors typically draw around six times their running current when starting, which often decides the genset size more than the running load. - Running a diesel genset for long periods below roughly 30 percent load risks wet stacking, so oversizing is not free. - Altitude and high ambient temperature reduce output; always check the derating table on the data sheet. ## kW vs kVA: what is the difference? kW (kilowatts) is the real power that does work, while kVA (kilovolt-amperes) is the total apparent power the alternator must carry, including the reactive current drawn by motors and transformers. The two are linked by power factor (PF): kW = kVA × PF. Three-phase generators are almost always rated at 0.8 PF, so a 500 kVA set delivers 400 kW. Single-phase portable units are often rated at 1.0 PF, which is why their kW and kVA figures match. The engine is sized for kW, because that is what it must turn, while the alternator is sized for kVA and current. For three-phase systems, current is calculated as: amps = kVA × 1,000 ÷ (1.732 × line voltage). The table shows typical generator sizes at 0.8 PF. | Rating (kW) | Rating (kVA at 0.8 PF) | Full-load amps at 400 V | Full-load amps at 480 V | Typical use | | --- | --- | --- | --- | --- | | 20 | 25 | 36 | 30 | Small shop, telecom site, pump station | | 50 | 62.5 | 90 | 75 | Small commercial building, site cabins | | 100 | 125 | 180 | 150 | Workshop, cold store, small factory line | | 200 | 250 | 361 | 301 | Medium factory, hotel, construction site | | 400 | 500 | 722 | 601 | Large commercial building, quarry plant | | 600 | 750 | 1,083 | 902 | Hospital wing, process plant | | 2,000 | 2,500 | 3,608 | 3,007 | Data center module, mine, utility support | ## Standby, prime or continuous: which rating do I need? Choose the rating by how many hours the generator will run and how steady the load is. ISO 8528-1 defines the categories below; individual manufacturers may add their own, such as a data center rating, so read the definition on the data sheet. | ISO 8528-1 rating | Annual hours | Load profile | Overload | Typical use | | --- | --- | --- | --- | --- | | Emergency standby power (ESP) | Up to about 200 h/yr | Variable, average up to 70% over 24 h | None | Backup during utility outages | | Limited-time running power (LTP) | Up to about 500 h/yr | Constant, up to 100% | None | Peak shaving, planned outages | | Prime rated power (PRP) | Unlimited | Variable, average up to 70% over 24 h | Typically 10% for 1 h in 12 h | Off-grid sites, construction, mines | | Continuous operating power (COP) | Unlimited | Constant, up to 100% | None | Base-load plants, parallel with grid | The same set carries a higher number on a standby rating than on a prime rating, often by about 10 percent. A "100 kW" standby set is therefore a smaller machine than a "100 kW" prime set. When comparing quotations, always compare at the same rating. ## How does diesel generator sizing work, step by step? A sound sizing exercise follows the same sequence for a 20 kW electric generator or a 2,000 kW plant: - **List every load** with its rated kW or kVA, voltage, phase and power factor. Use nameplates or measured data rather than breaker sizes. - **Separate running loads from starting loads.** Motors, compressors, pumps and elevators need extra kVA while starting; lighting, heaters and electronics do not. - **Decide which loads run together.** Apply realistic diversity, but treat life-safety and process-critical loads as always on. - **Plan the load sequence.** If the transfer switch or controller can bring loads on in steps, the genset only needs to absorb the largest step, not the total, at once. - **Check the largest step against transient performance.** The set must recover voltage and frequency within the limits the loads tolerate. ISO 8528 defines performance classes G1 to G4 for this, with G2 typical for commercial supplies and G3 for sensitive loads. - **Account for nonlinear loads.** UPS systems, VFDs and LED drivers create harmonics; alternators are often oversized for them. - **Apply site derating** for altitude, ambient temperature and air quality. - **Add a margin** of roughly 10–25 percent for growth, then choose the rating category for the duty. Manufacturer sizing software performs steps 2 to 7 in detail. A rough manual estimate is useful, but use the software or an electrical engineer for critical facilities. ## How do motor starting loads affect genset size? Motor starting is often the deciding factor in diesel generator sizing. A direct-on-line induction motor typically draws around six times its full-load current at start-up, at a low power factor, so a 50 kW motor can briefly demand several hundred kVA. If the generator cannot supply it, voltage dips deeply, contactors drop out and the motor may stall. Ways to reduce the genset size needed for motors: - Use soft starters or variable-frequency drives, which can cut starting kVA sharply. - Start the largest motor first, while the set is lightly loaded. - Stagger motor starts with timers in the controller. - Choose an alternator with a higher motor-starting capability; alternator frame size, not engine size, often limits starting kVA. ## Three-phase or single-phase generator? Most industrial and commercial gensets above about 20 kW are three-phase. A three-phase generator delivers power more smoothly, uses less copper for the same power and runs three-phase motors directly. Single-phase sets suit homes, small shops and light tools. Common three-phase voltages are 400/230 V at 50 Hz in Europe, China, much of Asia and Africa, and 480/277 V or 208/120 V at 60 Hz in North America. A 480 V generator for a US plant and a 400 V generator for export may share the same engine but need different alternator windings, governor speed and voltage settings, since a four-pole set runs at 1500 rpm for 50 Hz and 1800 rpm for 60 Hz. ## How does a generator produce electricity? A generator produces electricity by electromagnetic induction. The diesel engine spins the alternator rotor, which carries a magnetic field fed by an exciter; as this field sweeps past the stator windings it induces an alternating voltage in them. In a three-phase alternator the three stator windings are spaced 120 degrees apart, giving three overlapping voltage waves. The automatic voltage regulator adjusts excitation to hold voltage steady, and the engine governor holds speed, and therefore frequency, steady as load changes. ## Why is an oversized diesel generator a problem? An oversized set costs more to buy and can be damaged by running too lightly, so good diesel generator sizing aims for a set that is well loaded most of the time. Diesel engines that run for long periods below roughly 30 percent load may not reach full combustion temperature, so unburned fuel and soot collect in the exhaust, a condition called wet stacking. It leads to carbon buildup, oil dilution and blocked aftertreatment. If loads vary widely, two or more paralleled sets, sometimes called a twin pack, can be run to match demand. For prime power, a set loaded at around 50–80 percent of its prime rating most of the time is a common target. ## Choosing the engine behind the rating Genset builders select the engine first by its prime and standby output at 1500 or 1800 rpm. Six-cylinder engines such as the [6BTA genset engine rated 92 kW](/products/diesel-engines/cummins-type-6bta-diesel-engine-for-genset-and-marine-92-kw/) or the [6CTA8.3 family](/products/diesel-engines/cummins-type-6cta8-3-diesel-engine-8-3-l-80-210-hp/) are common for sets of roughly 100–250 kVA, while larger engines like the [19-liter KTA19 platform](/products/diesel-engines/cummins-type-kta19-c600-diesel-engine-448-kw-600-hp/) are a common basis for higher-output sets. Confirm the genset-specific rating, not a construction or marine rating, before matching an alternator. See more on the [diesel engines](/products/diesel-engines/) page, our [industrial diesel generator guide](/blog/industrial-diesel-generator-engines-guide/) and the [marine generator sizing guide](/blog/marine-generator-sizing-guide/) for vessels. MARS supplies genset engines with a test run, written report and video before dispatch. For help matching an engine to your load list, [send the details](/contact/) . ## Frequently asked questions How do I convert kW to kVA for a generator? Divide kW by the power factor. At the standard 0.8 power factor, 100 kW equals 125 kVA and 400 kW equals 500 kVA. What size generator do I need to run a 3 phase motor? It depends on the motor's starting method. For direct-on-line starting, the genset often needs two to three times the motor's kW in capacity or more, while a soft starter or VFD reduces this considerably; check with sizing software. Can I run a standby generator as prime power? Not at its standby number. If the set will run long hours, use its prime rating, which is typically about 10 percent lower, and keep the average load within the prime limits. What is the most common generator size for a small business? Many small commercial buildings use sets in the 20–100 kW range, but the right size comes only from a load list. Sizing by building area or by guesswork usually leads to over- or under-sizing. Does altitude affect diesel generator output? Yes. Thinner air reduces engine power, so most data sheets state full output only up to a given altitude and ambient temperature and list a derate above it. M **MARS Engineering & Sales Team** Written by the engineering and sales team at Shanghai Mars Industrial Co., Ltd., who specify, inspect and ship marine and industrial power equipment from Shanghai every week. ## Share - [Share on LinkedIn](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Fdiesel-generator-sizing-kw-kva%2F) - [Share on WhatsApp](https://wa.me/?text=Diesel+Generator+Sizing%3A+kW+vs+kVA%2C+Prime+and+Standby+Ratings+https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Fdiesel-generator-sizing-kw-kva%2F) - [Share by email](mailto:?subject=Diesel+Generator+Sizing%3A+kW+vs+kVA%2C+Prime+and+Standby+Ratings&body=https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Fdiesel-generator-sizing-kw-kva%2F) **Products mentioned** - [Cummins-Type 6BTA Diesel Engine for Genset & Marine (92 kW)](/products/diesel-engines/cummins-type-6bta-diesel-engine-for-genset-and-marine-92-kw/) - [Cummins-Type 6BT Diesel Engine for Genset & Marine (80-210 HP)](/products/diesel-engines/cummins-type-6bt-diesel-engine-for-genset-and-marine-80-210-hp/) - [Cummins-Type 6CTA8.3 Diesel Engine (8.3 L, 80-210 HP)](/products/diesel-engines/cummins-type-6cta8-3-diesel-engine-8-3-l-80-210-hp/) - [Cummins-Type KTA19-C600 Diesel Engine (448 kW / 600 HP)](/products/diesel-engines/cummins-type-kta19-c600-diesel-engine-448-kw-600-hp/) - [Doosan PU086TI Industrial Diesel Engine](/products/diesel-engines/doosan-pu086ti-industrial-diesel-engine/) ## Need a quotation? 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