In short. A power requirement that can be engineered has eleven parts: the application, the location, the existing supply, the running load, the largest transient load, the voltage, phase and frequency, the continuity requirement, the site conditions, the fuel autonomy, any redundancy and whether the need is temporary. Configuring a requirement means writing each down with its source and uncertainty, so the generator, panel and installation are selected from evidence rather than a kVA guessed over the phone.
Send your enquiry
Why the requirement comes before the generator
The commonest sizing error in Indian industry is to add up the nameplates of everything on site, apply a round margin and buy that kVA. The result is a set that runs at a quarter load for life, or one that trips every time the largest compressor starts because the total was right but the transient was never considered. A configured requirement separates those questions. It states what the set must carry continuously, what it must survive momentarily, what the loads can tolerate during a transfer, and what the site will do to the set’s rating. The generator is the output of that work, and the detailed method is under generator sizing. What follows is the checklist of inputs and why each one changes the answer.
The eleven inputs and what each one decides
| Input | What to record | What it decides |
|---|---|---|
| Application | Factory, hospital, data hall, pump station, construction site, event | Duty class, acceptable interruption, applicable standards |
| Location | Address, altitude, indoor or outdoor, urban or rural | Derating, noise limits, emission norms, access for delivery and service |
| Existing power system | DISCOM connection and contract demand, transformer rating, existing generators and panels | Transfer arrangement, paralleling scope, reuse of infrastructure |
| Running load | Measured demand in kW and power factor, or a load list with diversity | Base generator kW |
| Largest transient load | Biggest motor with starting method, UPS rating, transformer inrush, welding or medical imaging loads | Alternator size, voltage dip, step acceptance |
| Voltage, phase and frequency | 415 V three-phase or 230 V single-phase at 50 Hz; any 11 kV or 60 Hz need | Alternator winding, transformer need, switchgear class |
| Continuity requirement | Which loads must never break, which accept 10 to 30 seconds, which can wait | UPS, AMF timing, load shedding, number of sets |
| Site conditions | Ambient maximum, dust, humidity, space, ventilation, fuel delivery access | Derating, enclosure type, cooling, filtration |
| Fuel autonomy | Hours the set must run without refuelling, delivery lead time | Tank size, day-tank arrangement, storage compliance |
| Redundancy | N, N+1 or 2N expectation, maintenance cover | Number and size of sets, synchronising |
| Temporary power need | Duration, start date, whether the load will grow | Rent or buy, mobile or stationary set |
Getting the load figures right
Two load figures carry most of the weight. The running load should come from measurement wherever a supply exists: a month of DISCOM maximum demand data or a week of logging on the main incomer is worth more than any nameplate list. Where the site is new, build a load list with realistic demand and diversity factors rather than connected load, following the method under load inventory. The transient figure is the largest single step the set will see, which is usually the biggest direct-on-line motor but can be a UPS switching on with discharged batteries, a transformer being energised or a bank of lights after a transfer. For that step, record the kW, the starting method and whether it can be sequenced after the rest of the site is stable. A 55 kW motor started direct-on-line draws roughly six to seven times its full-load current for a second or two, and its effect on the alternator is covered under motor starting.
Sgen ≥ Prun / PF, checked against Sstart at the permitted voltage dip
Here Sgen is the generator rating in kVA, Prun is the running load in kW, PF is the load power factor, and Sstart is the starting kVA demanded by the largest transient. Worked example (illustrative): a workshop measures 180 kW running at 0.85 power factor, giving 212 kVA, so a 250 kVA set would carry the steady load at about 85 per cent. Its largest motor is 45 kW started direct-on-line, demanding around 300 kVA for a moment; whether the 250 kVA set holds voltage within the 15 to 20 per cent dip the site accepts depends on the alternator’s transient reactance and must be checked against the manufacturer’s motor-starting data, and if not, a soft starter or a larger alternator frame is the answer.
Loads that need an engineering review
- Any single motor above about 20 per cent of the intended generator kW, or several large motors that may start together.
- UPS systems and variable-speed drives, whose rectifiers distort current and can unsettle the AVR; see UPS and non-linear loads.
- Medical imaging, welding, induction heating or lifts with regenerative drives, all of which impose steps or reverse power.
- Sites above 1,000 metres or with summer ambient above 40 °C, where ambient temperature derating removes capacity the nameplate promises.
- Requirements for paralleling with the DISCOM, synchronising more than one set, or feeding at 11 kV.
- Critical loads where the generator is the second line behind a UPS, so that AMF timing, UPS autonomy and load restoration must be coordinated.
Writing the brief so it survives the quotation stage
A good requirement brief fits on a page and states, for every figure, whether it was measured, calculated from a list or assumed. It names the continuity consequence in plain terms: which production line stops, which ward is affected, how much product spoils per hour. It states the fuel autonomy in hours and the delivery lead time, because that fixes the tank and the arrangement discussed under fuel tank sizing. It gives the site constraints that decide the physical form: the space available, whether the set must be inside a building, the distance to the nearest neighbour for noise, and how a lorry and crane will reach the position. And it says what is unknown. A brief that admits the running load is estimated and the largest motor is yet to be confirmed produces a quotation with those items flagged for verification, which is far safer than a quotation that quietly assumes them.
Common mistakes
- Stating the connected load as the running load; the set is oversized by 50 to 100 per cent and suffers light-load problems for life.
- Omitting the largest motor and its starting method; the set carries the steady load and trips on the first compressor start.
- Specifying kVA without power factor; two suppliers quote different kW capacities for the same number and the comparison is false.
- Forgetting the site ambient and dust; the set delivers less than its nameplate in May and clogs its filters by June.
- Leaving the continuity requirement vague; the AMF delay, UPS and shedding scheme are designed to guesses and the critical load still drops.
What to send us
Send the completed requirement in whatever form you have it: a load list or metered demand data, the nameplate of the largest motor and any UPS, a note on voltage and phase, the site address with photographs of the proposed generator position, the hours of autonomy needed, and the continuity consequence of an outage. Mark anything you are unsure of. We will return a requirement summary with the generator range, panel and installation items it implies and the figures that still need confirming. Send your power requirement for engineering review.
Questions engineers ask
How much margin should I add to the running load?
Enough to keep the set between roughly 50 and 80 per cent loaded in normal operation, after allowing for confirmed expansion plans. A fixed 25 per cent margin on top of connected load almost always produces an oversized set; a margin on measured demand is the right basis.
Do I need to specify power factor?
Yes. Generator sets are rated in kVA at 0.8 power factor, so a 250 kVA set delivers 200 kW at that figure and its engine is sized accordingly. A site at 0.95 power factor with power factor correction is engine-limited before it is alternator-limited, and the quotation needs to reflect that.
What if I do not know the largest motor?
Say so, and we will identify it on a site visit or from the motor control centre schedule. It is the figure most likely to change the generator size, so it should never be assumed.
Does a temporary requirement need the same detail?
It needs the same load and transient figures, because a rental set trips on an unanticipated motor start exactly as a purchased one does. It needs less on installation and more on access, cable lengths, fuel responsibility and duration.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.