A generator has to do two different jobs. It must carry the load that runs for hours, and it must absorb the shock of the largest motor starting on top of that load without its voltage collapsing. Size for the first only and the compressor stalls the set; size for the second with no thought for the first and you buy an engine that spends its life at 20 % load, coking its exhaust. Getting both right takes ten minutes of arithmetic that most quotations skip.
Step 1: list what runs at the same time
Not everything in the building, only what the generator will actually supply, and only what will be on together. Walk the site with a clipboard, or better, log the incomer with a clamp meter for a week and read the true maximum demand. Write each load down as electrical input in kW with its power factor:
- Lighting and electronics: nameplate watts, power factor 0.9–1.0.
- Heaters, ovens, geysers: nameplate watts, power factor 1.0.
- Motors (pumps, compressors, fans, lifts, machine tools): shaft kW ÷ efficiency, power factor 0.8–0.9 at full load. The power and current calculator converts horsepower.
- Air conditioners: running input from the nameplate; note whether each is inverter type or fixed-speed, which matters for starting.
- UPS units, drives and other electronic loads: their input kVA, flagged separately because they distort the current (more below).
Add up the kW and divide by the overall power factor for the running kVA. Twenty kilowatts of mixed load at 0.8 is 25 kVA.
Step 2: the largest motor start
An induction motor started direct-on-line draws about six times its running current for a second or two. A 5.5 kW compressor motor that runs at 7 kVA demands 43 kVA while it accelerates. On the utility that is nothing; on a generator it is a step load that the alternator must supply from a standing start with its voltage regulator scrambling to keep up.
Alternators can supply a short overload of two to three times their continuous rating while the voltage dips 20–30 %, and most sets are specified with a motor-starting kVA on the datasheet. So the check is:
For the example: (17 + 43) ÷ 2.5 = 24 kVA. Here the running load governs, but with a 15 kW motor started direct-on-line inside the same 20 kW load the starting requirement would be about 49 kVA and dictate the size. That is when a soft starter or a variable-frequency drive on that one motor pays for itself several times over: it cuts the step to two or three times running current and lets the set stay one or two sizes smaller. Sequencing, starting the largest motor first while the set is unloaded, does the same for free if the process allows it.
The generator sizing calculator carries both requirements through the margin and derating below.
Step 3: understand the rating on the nameplate
Generator sets are rated in kVA at 0.8 power factor, so a 40 kVA set delivers 32 kW; the engine behind it is sized for the 32 kW plus the alternator's losses. ISO 8528-1 defines four ratings and sellers quote the most flattering one:
- Emergency standby (ESP): the maximum for the duration of an outage, with the average load held to 70 % and around 200 hours a year.
- Prime (PRP): unlimited hours at variable load averaging 70 %, with a 10 % overload allowed for an hour in twelve. Typically about 90 % of the standby figure.
- Limited-time (LTP) and continuous (COP): for peak-lopping and base-load duty respectively.
With load-shedding running to several hours a day, a Pakistani "standby" set is in prime duty by any honest definition. Size against the prime rating, and treat the standby figure as the overload margin it is.
Step 4: derate for the site
An engine is rated at sea level and 25 °C intake air. Thinner air at altitude and hotter air in a generator room both mean less oxygen per stroke and less power. Commonly quoted allowances for turbocharged diesels are about 3.5 % per 300 m above 900 m and about 2 % per 5.5 °C above 40 °C, with naturally aspirated engines and gas engines losing more; the manufacturer's curve for the specific set is the authority. Quetta at 1 700 m in a 45 °C summer takes roughly 11 % off a set that is fine in Karachi. The alternator derates for temperature too, since its windings have an insulation class with a maximum temperature.
The room matters as much as the map. A set in a small enclosed room with a poor exhaust run and no ducted cooling air can see intake temperatures 15 °C above ambient; ventilation is part of the sizing.
Step 5: add margin, round up, and then check you have not gone too far
Take the larger of the running and starting requirements, add 20–25 % for growth and the fact that everything ages, divide by the derating factor, and round up to the next standard size. Then compute the loading at the normal running load. Between about 30 % and 85 % of prime rating is healthy. Below 30 %, a diesel does not reach working temperature; unburnt fuel and carbon accumulate in the cylinders, turbocharger and exhaust (wet stacking), injectors foul, oil dilutes and life falls, and the set burns more fuel per unit than a smaller one would. Above 85 % there is nothing left for a second motor start or the next air conditioner.
If the arithmetic leaves the set lightly loaded because a single motor start dominates, fix the motor start rather than accept the oversized engine. If the load genuinely will grow, buy for today and plan a second set in parallel; two 30 kVA sets that share load are also a better outcome for reliability than one 60 kVA set.
Electronic loads and the alternator
UPS rectifiers, variable-frequency drives, LED drivers and computer power supplies draw non-sinusoidal current. On a stiff utility supply this hardly shows; on a generator, whose source impedance is many times higher, the harmonic currents distort the voltage waveform, which upsets the voltage regulator, other electronics, and the UPS itself, which may refuse to synchronise and stay on battery until it is flat. The usual precautions are to size the alternator at 1.5 to 2 times the kVA of the electronic loads (or specify a low-reactance alternator and a permanent-magnet excitation system), to fit input filters or 12-pulse rectifiers on large UPS units, and to talk to the UPS vendor about generator compatibility before, not after, the purchase.
The example in numbers
An office and small workshop in Lahore, 20 kW running at 0.8 with a 5.5 kW compressor started direct-on-line, sea level, 40 °C, 25 % margin:
- Running: 25 kVA. Starting: (17.4 + 43.1) ÷ 2.5 = 24.2 kVA. Running governs.
- Margin: 25 × 1.25 = 31.3 kVA. Derating: none.
- Standard size: 40 kVA prime. Loading at the running load: 62.5 %, healthy.
Fuel, for planning: a diesel set at 60–75 % load burns roughly 0.25–0.3 litres per kWh delivered, so this site at 20 kW uses about 5–6 litres an hour of load-shedding. That number, times the hours the utility is away, is what decides whether solar and batteries should carry the daytime load and leave the generator for evenings, which for most Pakistani commercial sites is now the cheaper arrangement.
References
- ISO 8528-1:2018, Reciprocating internal combustion engine driven alternating current generating sets — Part 1: Application, ratings and performance
- ISO 8528-5:2022, Part 5: Generating sets — transient performance classes G1–G4
- ISO 3046-1:2002, Reciprocating internal combustion engines — Performance — Part 1 — declared power and adjustment for site conditions
- IEC 60034-12:2016, Rotating electrical machines — Part 12: Starting performance of single-speed three-phase cage induction motors
- IEEE Std 446-1995 (Orange Book), Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications — generator application and motor starting