In short. An application assessment establishes what a generator must actually do before anyone compares brands or kVA ratings. It records the consequence of an outage, the acceptable restoration time, the hours and pattern of running, the loads and how they start, and the site conditions that derate equipment. The output is a written requirement and a shortlist of candidate solutions, not a nameplate picked from a catalogue.
Start with the process, not the generator
The first questions in an assessment have nothing to do with engines. What stops when the DISCOM supply fails, and what does an hour of that cost? How quickly must power return, in seconds for a server room or in minutes for a packaging line? How long must the set run, through a two-hour evening cut or a three-day line fault after a cyclone in coastal Andhra? Is the set for standby behind a reliable grid, for daily scheduled running where the utility supply is weak, or for a defined temporary duty at a project? These answers set the duty rating, the fuel autonomy, the transfer arrangement and the level of redundancy, and they belong in writing before any equipment is discussed.
Duty rating: the same kVA is not the same set
Manufacturers rate sets according to ISO 8528-1, and the headline kVA on two quotations may describe different permitted duties. The distinctions matter for both selection and warranty.
| Duty (ISO 8528-1) | Typical permitted use | Average load limit | Suits |
|---|---|---|---|
| Emergency standby (ESP) | Limited hours per year, variable load, no overload | Often around 70 percent of ESP over 24 hours | Buildings and plants with reliable grid |
| Prime (PRP) | Unlimited hours, variable load, 10 percent overload for short periods | Often around 70 percent of PRP over 24 hours | Weak-grid factories, long daily running, rental |
| Limited-time prime (LTP) | Limited hours per year at constant load up to the rating | 100 percent for the permitted hours | Scheduled peak-shaving or utility-curtailment running |
| Continuous (COP) | Unlimited hours at constant load | 100 percent | Base-load and off-grid plants |
A set quoted at 500 kVA standby may be a 450 kVA prime machine, so comparing it against a competitor’s 500 kVA prime set for daily running on a weak feeder is a false comparison. The assessment records the expected hours per year and load profile so that the correct rating column is used, with the exact limits taken from the manufacturer’s rating definitions.
Build the electrical picture
With the duty fixed, the assessment gathers the loads. A load inventory lists each significant item with its running kW, power factor, starting method and whether it must restart automatically after an outage. Motors, UPS systems, drives, lifts, chillers and medical imaging equipment each behave differently when the generator picks them up. Measurements outrank nameplates, and estimates are marked as estimates. The sequence in which loads come back matters as much as their total, because the largest transient, not the running load, usually decides the alternator size. For existing facilities a load study replaces remembered demand with recorded demand.
From inventory to a sizing band
Sizing is not a kW to kVA conversion. The chain runs from inventory and critical-load definition, through running kW and kVA at measured power factor, load sequence, the largest motor start, UPS and non-linear load content, acceptable voltage and frequency dip, site derating, duty rating and future capacity, to verification of the engine and alternator against the manufacturer’s transient data. The assessment ends with a band, for example a 400 to 500 kVA class subject to transient review, rather than a premature single model. The arithmetic is set out in generator sizing and the motor-start specifics in motor starting.
Site conditions that derate the set
Hyderabad sits at around 500 to 550 metres and sees 42 to 45 degree summers. Most engines are rated to a reference altitude and ambient and lose output beyond them; a common rule of thumb is a few percent per 300 metres above the reference altitude and a few percent per 10 K above the reference ambient, but the actual figures are engine-specific and must be read from the derating tables. Radiator airflow, room ventilation and enclosure design add further constraints. An assessment records altitude, maximum ambient, dust, humidity, coastal exposure, and the space available for the air path, so that the candidate set is checked at site conditions rather than at the test-bed rating. See ambient-temperature derating and altitude derating.
Assess the system around the set
A generator never works alone. The assessment looks at the transfer arrangement, whether a simple AMF panel, an ATS with bypass, or synchronising with other sets or the utility. It looks at the switchgear and distribution the generator will feed, the neutral and earthing arrangement, the fuel supply and refuelling logistics, the noise environment, and the emission norms that apply, which for new diesel sets up to 800 kW means CPCB IV+ compliance. For a legacy set being evaluated for retrofit or replacement, the assessment also considers whether an emission control device, a dual-fuel conversion or a new set is the sensible path.
Matching the situation to the pathway
| Situation | Assessment focus | Likely outcome |
|---|---|---|
| New factory or building | Load inventory, duty, transfer, site | Sizing band, installation planning, specification |
| Existing set trips on load | Load measurement, sequence, protection, cooling | Diagnostics, re-sequencing, or upsizing |
| Frequent long outages | Autonomy, refuelling, redundancy | Fuel system upgrade, second set, service agreement |
| High running cost | Load profile, litres per kWh, set loading | Right-sizing, multiple smaller sets, load management |
| Temporary project | Duration, access, distribution, fuel logistics | Rental package with temporary distribution |
| Old set and new emission rules | Condition, retrofit compatibility, lifecycle cost | RECD, dual fuel or CPCB IV+ replacement |
What the assessment delivers
The written output states the requirement in engineering terms, the duty rating, the running and transient loads with their confidence, the site conditions and derating, the transfer and distribution interfaces, the fuel autonomy target, the applicable regulations, and the information still missing. It names candidate solutions as a class and lists the verifications still needed, such as a manufacturer transient check or a site survey. Equipment selection then proceeds from evidence, and the same document becomes the basis for installation planning and commissioning acceptance.
Common mistakes
- Comparing quotations by headline kVA without checking the duty column; a standby-rated set is bought for daily prime running and the warranty is voided within a year.
- Adding up nameplate ratings and applying a diversity guess; the set is either twice the size needed or trips on the first chiller start.
- Assessing at test-bed ratings and ignoring a 44 degree ambient and a tight generator room; the set delivers 85 percent of nameplate when it is needed most.
- Leaving the transfer scheme and neutral arrangement for later; the ATS, cables and earthing arrive as a surprise cost and a commissioning problem.
- Presenting estimates as measured demand; the uncertainty is hidden and the design carries it unknowingly.
What to send us
Send a description of the process and the cost or risk of an outage, the acceptable restoration time, the hours per day the set is likely to run, a load list with any measurements or utility bills, the single-line diagram if one exists, the site location with altitude and the space available, and photos of the existing switchboard and any existing set. Mark estimates as estimates. The application engineering enquiry is the place to send it, and our generator services overview shows how the assessment leads into the later stages.
Questions engineers ask
What is the difference between standby and prime rating?
Standby rating applies to limited hours per year on variable load with no overload capability, for sites with a reliable grid. Prime rating permits unlimited hours on variable load with a short-term overload allowance. The same engine and alternator usually carry a lower prime figure than standby, so the duty must be known before the kVA is compared.
How much bigger than my load should the generator be?
There is no single margin. The running load is checked against the duty rating with an allowance for growth, but the largest motor start or UPS inrush usually decides the alternator size, and site derating reduces what the set can deliver. Typical outcomes land at 1.25 to 2 times the running kW depending on the transients.
Does the assessment tell me which brand to buy?
It tells you the class of set, the duty, the transient performance required and the site constraints. Brand and model follow from checking candidate sets against those requirements with manufacturer data. We supply and service multiple engine makes and recommend on fit, not on label.
Can I do an application assessment for an existing generator?
Yes, and it is the right first step when a set trips, runs uneconomically or faces a new emission requirement. The assessment measures the real load, checks duty against actual hours and reviews the site, then recommends diagnostics, re-sequencing, retrofit or replacement.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.