Understanding kW, kVA, and Power Factor
Generator output is stated in both kW, the real working power, and kVA, the apparent power. They are related by the power factor, commonly 0.8 for generator sets, so a 100 kW unit is typically rated around 125 kVA. Size by the kW your load actually needs, and pay attention to kVA when your load includes many motors or other inductive devices. Confusing the two leads to buying a generator that looks big enough on paper but falls short in practice, or paying for capacity you will never use.
Prime vs Standby Rating
A generator's rating depends on how it will be used. Standby rating is for emergency backup, intended to run only during outages, and allows the highest short-term output. Prime rating is for continuous operation as the main power source and is lower because the set must sustain it indefinitely. There is also continuous rating for constant unvarying loads. Buying a standby-rated unit for daily prime duty will overwork it, while a prime-rated set makes an excellent, hard-working buy. Always confirm which rating the nameplate figure represents before you compare prices.
Engine Hours and Overall Condition
Engine hours matter, but a diesel generator engine is built for a long life, often many thousands of hours before major service. As with compressors, how those hours were accumulated matters: steady prime duty is gentler than frequent cold starts. Inspect the engine for oil and coolant leaks, check the exhaust for excessive smoke, and look at the general cleanliness and maintenance of the set. Examine the alternator, control panel, wiring, and radiator. A tidy, well-kept unit with consistent service points to an owner who ran it responsibly.
Insist on a Load Bank Test
The single most important test for a used generator is a load bank test, which applies a controlled electrical load so you can watch the set perform under real conditions. Idling or no-load running proves almost nothing. Under load, confirm the generator holds voltage and frequency steady, reaches its rated output, and runs without excessive smoke, surging, or overheating. A load bank test also burns off wet stacking in a lightly used diesel. If a seller cannot or will not load test, treat the rated output as unproven and inspect with extra caution.
Tier Emissions and Fluid Checks
Diesel generators are built to EPA Tier emissions standards, from older Tier 2 and Tier 3 engines through Tier 4 Final with aftertreatment. The tier affects where a unit can be used and what emissions equipment it carries, so confirm the engine's tier and whether it suits your location and application. Before purchase, check the engine oil for correct level and clean condition, inspect coolant and look for signs of oil in it, and check fuel system condition. Clean fluids and a known emissions tier round out the picture of a generator worth buying.
Frequently asked questions
What is the difference between prime and standby generator ratings?
Standby rating is for emergency backup during outages and permits the highest short-term output. Prime rating is for continuous use as the main power source and is lower because the set must sustain it indefinitely. Match the rating to how you plan to run the generator.
How do I convert kW to kVA for a generator?
Divide kW by the power factor, usually 0.8 for generator sets, to get kVA. So a 100 kW generator is about 125 kVA. Size the unit by the real kW your load requires and watch kVA when the load is heavily inductive.
Why is a load bank test so important?
A load bank test applies real electrical load so you can verify the generator holds voltage and frequency, reaches rated output, and runs clean. A unit can idle perfectly yet fail under load, so no-load running does not prove it is sound.
How many engine hours are acceptable on a used generator?
Diesel generator engines are built for thousands of hours, so a few thousand hours on a well-maintained set is often modest. Evaluate hours alongside how the unit was used, its service records, and especially its performance under a load bank test.