In short. Most generator purchases that go wrong were decided wrongly before the order was signed. The set is sized from a total of connected loads and ignores motor starting. Quotes are compared that do not include the same scope. The rating is wrong for the duty, or nobody checked the site, the emission and noise compliance, or the changeover arrangement. The ten mistakes below are the ones our service engineers see most often. Each has a simple check that prevents it.
Who this page is for: anyone about to buy a new or used diesel generator from 7.5 kVA to 2,000 kVA, including first-time buyers for homes, shops and offices, purchase teams in factories and hospitals, and consultants preparing a specification.
Key facts
- A three-phase set’s kW rating is its kVA rating × 0.8. A 125 kVA set delivers 100 kW.
- Direct-on-line motors draw 6 to 7 times their running current while starting. One large pump or compressor can set the generator size.
- A healthy diesel set should normally run at about 50–80% of its rating. Long periods below 30% cause wet stacking.
- Generator sets supplied in India must meet current central emission and noise rules, including type approval and an acoustic enclosure for the size ranges covered.
- New-set quotations commonly exclude GST, transport and installation. Unless the scope is written out, two quotes are rarely comparing the same thing.
The ten mistakes
Mistake 1: Sizing from the total of connected loads
What happens: the buyer adds up every nameplate in the building and buys a set to match. In most buildings not everything runs at once, so the set ends up badly oversized, runs at light load and suffers wet stacking. The opposite also happens: a list that leaves out the borewell pump or the lift gives a set too small to start them.
The check: build a load list with each load’s running kW, how many run together (diversity), and which ones start with a surge. Our electrical load list builder and maximum demand calculator do this properly.
Mistake 2: Ignoring motor starting and non-linear loads
What happens: the set carries the running load comfortably but the lights dip, contactors chatter and the pump fails to start. UPS systems and variable-speed drives can make the voltage unstable or the AVR hunt.
The check: list every motor above about 5 kW with its starting method, and every UPS and drive. Check the largest motor start against the alternator’s motor-starting capability. The motor starting calculator and our UPS and non-linear loads page explain the limits.
Mistake 3: Choosing the wrong rating for the duty
What happens: a set rated for standby (emergency use, limited hours, variable load) is put into daily prime-power duty at a site with poor grid supply. It is overloaded against its true continuous capability and wears out early. The opposite mistake, buying continuous-rated capacity for a set that runs 50 hours a year, wastes capital.
The check: estimate annual running hours and the load pattern, then choose standby, prime or continuous rating to match. See standby, prime and continuous power.
Mistake 4: Comparing quotes that do not cover the same scope
What happens: the cheapest-looking quote turns out to exclude the AMF panel, battery, cables, earthing, exhaust extension, foundation, transport, unloading, installation, commissioning or the first service. The extras arrive later, piece by piece.
The check: send every supplier the same scope sheet and ask them to mark each line included, excluded or optional. Ask for GST, transport and installation to be shown separately. Compare like with like before comparing totals.
Mistake 5: Not checking emission and noise compliance
What happens: a set without valid type approval or a compliant acoustic enclosure is bought, often at a tempting price as old stock or an import. It cannot be legally installed, fails inspection, or must be retrofitted.
The check: ask for the type-approval certificate for the engine and set model, confirm the enclosure meets the noise rule, and check the rating plate and serial number against the documents. See India generator emissions and generator noise limits in India.
Mistake 6: Buying before checking the site
What happens: the set arrives and will not pass through the gate, cannot be lifted onto the terrace, has no space for door opening and service, or sits where its hot air and exhaust recirculate. Overheating trips follow in the first summer.
The check: survey access (gate width, crane reach, floor loading), clearances on all sides, airflow paths, exhaust discharge point, fuel delivery route and drainage before finalising the model. See generator site planning and site survey.
Mistake 7: Forgetting the changeover and protection
What happens: the generator is bought, but the changeover is left to an electrician to “manage”. The breaker is sized wrongly, the neutral is switched inconsistently, or there is no interlock between grid and generator. The result can be backfeed into the grid, nuisance tripping or a dead building when the AMF fails.
The check: decide manual changeover, ATS or AMF at the purchase stage. Size breakers and cables with the set, and confirm the mechanical and electrical interlocks. See AMF panels and the breaker, changeover and cable size chart.
Mistake 8: Single-phase and three-phase mismatch
What happens: a single-phase set is bought for a building with three-phase motors, or a three-phase set feeds badly unbalanced single-phase loads. The second case overheats one phase of the alternator while the meter shows plenty of spare kVA.
The check: map loads to phases. Keep imbalance within the alternator maker’s limit, often about 20–25% between the most and least loaded phases (measured as the spread between the highest and lowest phase current; the log book’s 10% figure is the deviation from the average, a different and stricter measure), unless the set is specified for unbalanced duty. See single-phase and three-phase generators.
Mistake 9: Ignoring service support and spares
What happens: a set from a make with no local service presence is bought on price. The first breakdown takes weeks to fix because parts and trained people are far away.
The check: before buying, confirm who will service the set where it is installed, how filters and common wear parts will be obtained, and whether an AMC can be arranged. Our service engineers cover all major makes across Telangana, Andhra Pradesh and every district within about 300 km of Hyderabad.
Mistake 10: Skipping commissioning and handover
What happens: the set is started, run for ten minutes off load and signed off. Faults in wiring, protection settings and cooling appear months later under real load, often outside the warranty conditions.
The check: insist on a documented commissioning with a load test, protection checks, an AMF transfer test and a handover file (manuals, test sheet, warranty terms, approvals). See commissioning and commissioning checklist.
Worked example (illustrative): sizing a small office building
Loads: lighting, computers and small power 18 kW; six 2-ton split air conditioners at about 2.2 kW each; one lift with a 7.5 kW motor; one 7.5 kW (10 HP) borewell pump started direct-on-line.
- Connected total: 18 + (6 × 2.2 = 13.2) + 7.5 + 7.5 = 46.2 kW. As kVA at 0.8 power factor, that is 57.75 kVA. The “add it all up” buyer would stop here.
- Apply diversity: lighting and air conditioners at 80% simultaneous use gives 18 × 0.8 = 14.4 kW and 13.2 × 0.8 = 10.56 kW. Lift and pump are taken at 100%. Running demand = 14.4 + 10.56 + 7.5 + 7.5 = 39.96 kW, about 50 kVA.
- Check the pump start: running kVA = 7.5 ÷ (0.88 efficiency × 0.85 power factor) ≈ 10.0 kVA. Starting direct-on-line at about 6 times gives roughly 60 kVA for a second or two, at low power factor, on top of the running load.
- Select: a 62.5 kVA set (50 kW) runs at 39.96 ÷ 50 ≈ 80% on peak demand and lower at normal times, at the top of the healthy band. A 60 kVA direct-on-line surge on top of about 50 kVA of running load is more than a 62.5 kVA alternator can take without a deep voltage dip, so fit the pump with a star-delta or soft starter, which cuts the surge to about one third, roughly 20 kVA. That is usually the cheaper fix. If the pump must stay direct-on-line, the next size up, 82.5 kVA (66 kW, about 61% load), is the alternative, confirmed against the alternator’s motor-starting data.
- Avoided mistakes: not 50 kVA from the running figure alone (no margin for the pump start), not a larger set picked “to be safe” without a load list, and the starter decision made before purchase, not after the first failed start.
Pre-purchase checklist
Complete this before you approve a purchase order.
| Area | Question | Answer / evidence |
|---|---|---|
| Load | Load list with running kW, diversity and peak demand prepared? | |
| Load | Largest motor, its kW and starting method? | |
| Load | UPS, VFD and other non-linear loads listed? | |
| Duty | Expected running hours per year and load pattern? | |
| Duty | Rating chosen: standby, prime or continuous? | |
| Electrical | Single-phase or three-phase; phase balance checked? | |
| Electrical | Changeover type: manual, ATS or AMF? | |
| Electrical | Breaker, cable and earthing sizes agreed? | |
| Compliance | Type-approval certificate seen for this model? | |
| Compliance | Acoustic enclosure compliant with the noise rule? | |
| Site | Access, lifting and floor loading checked? | |
| Site | Clearances, airflow and exhaust discharge point agreed? | |
| Site | Foundation or plinth design ready? | |
| Fuel | Autonomy hours required and tank size? | |
| Quotes | Same scope sheet sent to all suppliers; inclusions marked? | |
| Quotes | GST, transport and installation shown separately? | |
| Support | Who services the set at this location; spares availability? | |
| Handover | Commissioning load test and handover file included? |
Exceptions and cautions
- Data centres, hospitals and other critical sites need formal studies (transient response, fault level, redundancy) that go beyond this checklist. See our generator sizing guidance.
- Diversity factors depend on the building and its use. The 80% in the worked example is an illustration, not a rule.
- Regulations on emissions, noise and electrical approval change. Confirm current requirements at the time of purchase.
- For a used or refurbished set, add history, running hours, inspection report and load-bank test results to the checklist.
A quick way to rank the risks
| Mistake | How often we see it | Cost if it happens | Effort to prevent |
|---|---|---|---|
| Sizing from connected load | Very common | High: wrong set for its life | Low: a load list |
| Motor starting ignored | Common | Medium: starter or larger set | Low |
| Wrong duty rating | Occasional | High: early wear | Low |
| Unequal quote scopes | Very common | Medium: surprise extras | Low: one scope sheet |
| Compliance not checked | Occasional | Very high: cannot install | Low |
| Site not checked | Common | High: overheating, access cost | Medium: a survey |
| Changeover forgotten | Common | High: safety and reliability | Low |
| Phase mismatch | Occasional | Medium | Low |
| No local support | Occasional | High: long downtime | Low |
| No commissioning | Common | Medium to high | Low |
Common mistakes after purchase
- Keeping the old changeover panel because “it was working”.
- Not recording the set’s serial numbers and documents, then struggling at inspection or resale.
- Starting the warranty clock with a sloppy commissioning that the maker later disputes.
- Leaving staff untrained on what the controller alarms mean.
What to send us
For an independent selection, send your load list (or photographs of the main distribution board and motor nameplates), how many hours a year you expect to run, the site address and photographs of the proposed location and access route. If you already have quotations, send the scope pages and we will help you compare them line by line. Use the generator sales enquiry or WhatsApp 98852 01608. For pricing, use the Get price option on the enquiry.
Questions engineers ask
How do I choose the right generator size?
List each load’s running kW, apply realistic diversity, add the starting surge of the largest motor, and choose a set that runs at about 50–80% of its rating with that motor start confirmed against the alternator’s capability.
Should I buy a bigger generator for future expansion?
Only if the expansion is planned and near. Years at light load damage a diesel engine. It is often better to plan space and cabling for a second set later.
What should a generator quotation include?
The set, enclosure, AMF or changeover, battery, cables, earthing, exhaust, foundation, transport, unloading, installation, commissioning and the first service, each marked included or excluded, with GST shown separately.
What is the difference between standby and prime rating?
Standby rating is for emergency use with limited annual hours. Prime rating is for sets that are the main supply, running unlimited hours at variable load. Choosing the wrong one shortens engine life or wastes capital.
Do I need approval to install a diesel generator?
Generally yes. Electrical installations above certain limits need electrical-inspectorate approval, and the set must carry valid emission type approval and meet the noise rule. Confirm what applies to your site and set size.
Is a cheaper imported or old-stock generator a good idea?
Only if it carries valid Indian type approval, a compliant enclosure and local service support. Without these it may not be legally installable.
Related pages
- Generator sizing
- 12 generator myths that cost owners money
- DG set approvals in Telangana and Andhra Pradesh
- Diesel generator price guide
- Rent or buy
- Used and refurbished generators
- Generator site planning checklist
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 5 November 2026.