In short. Most generator failures we see in the field do not come from bad machines. They come from beliefs that sound sensible and are wrong: that a bigger set is safer, that a weekly off-load run keeps a standby set healthy, that diesel keeps for ever, that kVA and kW are the same thing. This page takes the twelve myths that cost owners the most and sets out what is actually true, and why, in engineering terms.
Who this page is for: owners, buyers, facility managers and maintenance staff who make decisions about generators without being engine or alternator specialists. It is the first reference to give to anyone who says “we have always done it this way”.
Key facts
- A diesel engine is built to work. Most of the damage done to standby sets is done by running them too lightly or not running them at all, not by heavy use.
- A 100 kVA set at the usual 0.8 power factor delivers 80 kW of real power. The kVA figure is not the kW you can connect.
- A direct-on-line motor draws 6 to 7 times its running current while starting, at a poor power factor. Motors, not lights, usually decide the generator size.
- Stored diesel in a typical Indian canopy tank should be used, tested or polished within 6 to 12 months.
- Most makers ask for the first oil and filter service after the first 50 to 250 hours, depending on the engine, not “when it gives trouble”.
The twelve myths
Myth 1: “A bigger generator is always the safer choice.”
The fact: an oversized set is a less reliable set. Diesel engines need enough load to reach full combustion temperature. Below roughly 30% of rated load, unburnt fuel and oil collect in the exhaust and on piston rings. This is known as wet stacking, and it shows as oily exhaust, glazed cylinder liners and rising oil consumption.
Why: combustion pressure and temperature fall with load. At light load the piston rings are not pressed hard enough against the liner to seal, and the cylinder walls glaze. The right size is the one that runs at about 50–80% of its rating for most of its life, with headroom for motor starting. Our page on light loading and wet stacking covers this in depth.
Myth 2: “A standby set that is never used will be ready when we need it.”
The fact: an idle set deteriorates. Batteries self-discharge and sulphate, fuel absorbs water, seals harden, coolant loses its inhibitors and alternator windings absorb moisture.
Why: nothing in a standby set is static. A lead–acid battery off charge loses a few percent of its charge every month and sulphates if left discharged, and a failed charger often goes unnoticed until the set will not crank. Seals stay supple only when wetted by circulating oil. Sets need a planned exercise and maintenance routine that matches their duty, even if they rarely see an outage.
Myth 3: “Running it off-load for ten minutes a week keeps it healthy.”
The fact: short off-load runs do more harm than good if they are the only exercise the set gets.
Why: ten minutes off load never brings the engine to full operating temperature. Water vapour from combustion condenses in the crankcase and exhaust, fuel dilutes the oil, and carbon builds up. A better routine is a weekly or fortnightly run on at least 30–40% load, long enough to reach stable temperature, with a longer monthly run on real building load, or on a load bank where the site load is small. See load bank testing.
Myth 4: “Diesel keeps for ever in the tank.”
The fact: diesel ages. It collects water, grows microbes and oxidises into gums and sludge.
Why: daily heat cycling makes the tank breathe in humid air, which condenses. Microbes thrive at the fuel–water boundary at Indian temperatures. Plan to use, test or polish fuel within six to twelve months. Our diesel stock planning guide shows how much to hold and how to rotate it.
Myth 5: “kVA and kW are the same thing.”
The fact: kVA is apparent power; kW is real power. For a generator rated at 0.8 power factor, kW = kVA × 0.8.
Why: the engine is sized for kW (real power that does work), and the alternator for kVA (the combined current it must carry). A 62.5 kVA set gives 50 kW. Connecting 62.5 kW of heaters to it overloads the engine even though the kVA figure looks fine. See the kW and kVA calculator.
Myth 6: “If the generator’s kVA is bigger than the motor’s, it will start the motor.”
The fact: starting kVA, not running kVA, decides the outcome.
Why: a direct-on-line motor draws a surge of 6 to 7 times its full-load current at a power factor near 0.3. The alternator’s voltage dips and, if it falls too far, contactors drop out and the motor stalls. Star-delta, soft starters and variable-speed drives cut this surge sharply. The motor starting calculator lets you check a real case.
Myth 7: “Any engine oil will do, and topping up with whatever is in the store is fine.”
The fact: diesel engines need the oil grade and service class the maker specifies, changed by running hours or by time, whichever comes first.
Why: modern low-emission engines run hotter, at higher pressures and with exhaust gas recirculation on some models, which loads the oil with soot. An oil without the right detergent and anti-wear package lets soot thicken the oil and wear the valve train. Mixing grades and brands dilutes the additive package. See our engine oil and coolant guide.
Myth 8: “Black smoke is normal for a diesel.”
The fact: a healthy, correctly loaded diesel generator shows little or no visible smoke after the first few seconds of start-up or a sudden load step.
Why: persistent black smoke means fuel is not burning fully: a choked air filter, overload, worn injectors, a faulty turbocharger or wrong injection timing. It also means wasted fuel and, on sets fitted with emission control, a faster-blocking filter. See black smoke from a generator.
Myth 9: “A silent canopy means the set can go anywhere, even indoors.”
The fact: an acoustic enclosure reduces noise. It does not remove the need for cooling air, exhaust discharge, clearance and safe fuel handling.
Why: a 250 kVA set rejects a large amount of heat through its radiator and needs a steady flow of fresh air; closed rooms and narrow alleys recirculate hot air until the set trips on high temperature. Exhaust gas discharged near windows or air intakes is a health hazard. Siting needs engineering; see generator room ventilation and generator site planning.
Myth 10: “Earthing the generator body is enough.”
The fact: a generator needs both body (equipment) earthing and correct neutral earthing, designed together with the building’s earthing system.
Why: body earthing protects people from touching a live frame. Neutral earthing gives fault current a defined return path so protection devices trip. A missing or doubled neutral earth causes nuisance trips, shock risk or protection that does not operate. See DG set earthing, step by step.
Myth 11: “Used generators are always unreliable.”
The fact: a used set’s reliability depends on its history, its inspection and how it was prepared for sale, not on its age alone.
Why: a well-maintained engine at moderate hours, inspected and tested on a load bank, can give years of service. A low-hours set that stood neglected may be in worse shape. At Reddy Generators every used or refurbished set is load-bank tested before sale; refurbished sets carry a six-month warranty (excluding rubber and plastic parts, consumables, fuel damage and overload), and used sets sold as-is come with an inspection report. See used and refurbished generators.
Myth 12: “The generator can wait for service until something goes wrong.”
The fact: waiting for a fault is the most expensive way to maintain a generator, because the fault usually shows itself during an outage.
Why: filters, belts, hoses, coolant and batteries wear on a predictable curve. Replacing them on schedule costs a fraction of a breakdown call, lost production and secondary damage, such as a blocked radiator causing a cracked cylinder head. A maintenance plan based on running hours and calendar time keeps the set ready. See generator maintenance planning and annual maintenance contracts.
Worked example (illustrative): two myths on one site
Site: a commercial building with 40 kW of running load, which includes a 7.5 kW direct-on-line water pump. The owner, believing “bigger is safer”, wants a 250 kVA set.
- 250 kVA at 0.8 power factor = 200 kW engine rating.
- Normal load as a share of rating: 40 ÷ 200 = 20%. That is in the wet-stacking zone for most of the set’s life.
- Correct sizing approach: running load 40 kW ÷ 0.8 = 50 kVA, plus headroom for the pump start and future growth.
- Pump starting check: 7.5 kW motor, efficiency 0.88, power factor 0.85, gives running kVA of 7.5 ÷ (0.88 × 0.85) ≈ 10.0 kVA. Starting direct-on-line at about 6 times, that is around 60 kVA for a second or two, at low power factor.
- A 60 kVA surge on top of the running load is too much to expect from a 62.5 kVA alternator without a deep voltage dip. An 82.5 kVA set (66 kW, about 61% load), checked against the alternator’s motor-starting data, handles the pump direct-on-line. Alternatively, a star-delta starter cuts the surge to about one third, roughly 20 kVA, and a 62.5 kVA set (50 kW, 80% load) becomes workable.
- Result: a set in the 62.5–82.5 kVA class runs at about 60–80% load, starts the pump and avoids wet stacking. The 250 kVA proposal would have cost more, burnt more fuel per useful kWh and needed load-bank runs to stay healthy.
Myth versus fact: quick reference
| # | Myth | Fact | What to do instead |
|---|---|---|---|
| 1 | Bigger is safer | Light load damages diesels | Size for 50–80% normal load |
| 2 | Idle sets stay ready | Idle sets deteriorate | Planned exercise and maintenance |
| 3 | 10 minutes off load is enough | Short off-load runs cause condensation and fuel dilution | Longer runs, real load, load bank |
| 4 | Diesel keeps for ever | Diesel ages in 6–12 months | Turn over, test, polish |
| 5 | kVA equals kW | kW = kVA × 0.8 | Size the engine in kW |
| 6 | Bigger kVA starts any motor | Starting kVA decides | Check motor starting; use soft starting |
| 7 | Any oil will do | Grade and class matter | Use the maker’s specification |
| 8 | Black smoke is normal | It signals a fault or overload | Diagnose the cause |
| 9 | Canopy sets go anywhere | Air, exhaust and clearance still needed | Engineer the site |
| 10 | Body earth is enough | Neutral earthing is also needed | Design earthing as a system |
| 11 | Used sets are unreliable | History and testing decide | Buy load-bank tested sets |
| 12 | Service when it fails | Failure comes at the worst moment | Hour- and calendar-based maintenance |
Exceptions and cautions
- Some applications deliberately run sets at light load for part of the time, such as redundant N+1 plants. These are managed with load-bank runs or load-dependent start and stop, not ignored.
- Exercise routines must follow the maker’s manual and the site’s own rules, especially in hospitals and fire-safety installations.
- The 0.8 power factor rating is standard for three-phase sets; some single-phase and special sets are rated differently. Check the rating plate.
- A worked example is a planning illustration. A real selection must use the actual alternator and engine data for the set being considered.
Common mistakes that grow out of these myths
- Buying for “future expansion” that never comes, then running the set at 15% load for ten years.
- Recording weekly test runs in the log book without ever putting the set on load.
- Choosing a set by kVA alone, without listing the motors and their starting methods.
- Letting a store-room oil drum decide the engine oil.
- Treating smoke, oil drips and slow cranking as normal “character” of an older set.
What to send us
If you suspect any of these myths has shaped your installation, send the set rating and make, the typical and peak load, a list of the motors with their starting methods, monthly running hours, and photographs of the rating plate and log book. Use the service and AMC enquiry, or the generator sales enquiry if you are choosing a new set, or WhatsApp 98852 01608. A service engineer will tell you plainly what needs to change.
Questions engineers ask
Is it bad to run a diesel generator at low load?
Yes, if it happens for long periods. Below about 30% load, unburnt fuel and oil build up in the exhaust and on the piston rings (wet stacking). Size the set correctly, or use periodic load-bank runs.
How often should a standby generator be run?
Typically weekly or fortnightly, on at least 30 to 40% load and long enough to reach stable operating temperature, with a longer monthly run on real load or a load bank. Follow the maker’s manual and any rules specific to your site.
Why is a 100 kVA generator only 80 kW?
Three-phase sets are rated at 0.8 power factor, so real power in kW is 80% of the kVA figure. The engine is sized for kW, the alternator for kVA.
Is black smoke from a generator normal?
Only for a moment at start-up or during a large load step. Continuous black smoke points to overload, a blocked air filter, injector or turbocharger faults, and should be diagnosed.
Can a silent generator be installed indoors?
Only with engineered ventilation, exhaust discharge to a safe outside point, fire and fuel safety measures and access for maintenance. The canopy reduces noise; it does not remove these needs.
Are second-hand generators worth buying?
A used set with a known history, a full inspection and a load-bank test can be a sound choice. Avoid sets sold without testing or with no record of hours and maintenance.
Related pages
- Light loading and wet stacking
- 10 mistakes to avoid before you buy a generator
- Generator sizing
- Motor starting on generators
- Diesel stock planning for standby generators
- Used and refurbished generators
- Generator maintenance planning
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 5 November 2026.