How to size a diesel generator set: a practical guide
Most generator problems are decided long before the set is built. A unit that is too small runs hot, trips on motor starting and wears out early; a unit that is far too large runs lightly loaded, glazes its cylinders and costs more than the project needed. The difference is a short sequence of questions, answered in the right order.
Start from the load, not from the generator
Write down every load the set has to carry, with its running power and the way it starts. Then add them up twice: once for the continuous load, and once for the worst moment — the largest motor starting while everything else is already running.
Power is quoted in kVA and in kW. The ratio between them is the power factor, and diesel generator sets are normally rated at 0.8: a 500 kVA set delivers 400 kW. Motors, transformers and uninterruptible power supplies are all specified in kVA; heaters, lamps and resistive loads in kW. Mixing the two units is the most common reason a set arrives undersized.
Choose the rating: prime, limited-time or standby
The same engine can be sold at three different numbers because the ratings describe different working lives. ISO 8528-1 defines them:
| Rating | What it promises | Typical use |
|---|---|---|
| PRP — prime power | Unlimited hours at a variable load, 10 % overload for one hour in twelve | A site with no reliable grid, or a set that carries the load every day |
| LTP — limited-time power | Up to 500 hours a year, no overload | Peak shaving, seasonal demand, planned outages |
| ESP — emergency standby | Emergency supply only, no overload, a few hundred hours a year | Hospitals, data centres, standby duty behind a mains supply |
Buying a standby rating for continuous duty is the second most common sizing error. If the set will run every working day, it needs a prime rating, whatever the price list says.
Settle 50 or 60 Hz first
Frequency is fixed by the engine speed and the alternator, not by a setting. A four-pole alternator gives 1500 rpm at 50 Hz and 1800 rpm at 60 Hz, and the two are different machines. A set built for one frequency cannot be moved to the other without changing the alternator, the engine governor setting and often the voltage.
Tell us where the set will work and which grid it feeds. Equipment nameplates carry the frequency, and a mismatch shows up as wrong motor speed, overheating and failed electronics.
Derate for altitude and temperature
Both the engine and the alternator lose capacity as the air thins and warms. Engine makers publish derating curves; typical figures for a naturally aspirated engine are about 4 % per 500 m above 1000 m, and about 2 % per 5.5 °C above 25 °C. Turbocharged engines lose less, but they still lose.
The alternator derates too, usually around 3 % per 500 m. A set selected at sea level and installed at 2000 m in a 45 °C plant room can end up 15–20 % below its nameplate, which is exactly the margin a project needs on a hot afternoon.
Account for motor starting and step loads
A squirrel-cage motor draws six to eight times its rated current when started direct on line. The generator sees that as a sudden kVA demand and its voltage dips; the usual acceptance limit is a dip of 15–20 % that recovers within a second or two.
Three things keep that under control: start the largest motor first and the others in sequence; use a soft starter, star-delta or a variable frequency drive on the big loads; and keep any single step below roughly 25–30 % of the set rating unless the alternator was chosen for heavier blocks.
Choose the alternator for the load, not for the price
Brush alternators such as the STC series are simple and inexpensive, and they suit steady loads where maintenance access is easy. Brushless machines such as the EL series hold voltage better under a changing load, need no brush or slip-ring service and are the better choice for motor starting and for unattended sites. Where the load includes large motors or the set has to sustain a short circuit for discrimination, a permanent magnet exciter is worth asking for.
A sizing worksheet
- Continuous load in kW and in kVA, and the power factor
- Largest motor: rated kW, starting method, starting current
- Any single load that is switched on in one step
- Altitude, ambient temperature and whether the room is ventilated
- Hours per year and the load profile — this decides the rating
- Voltage, frequency, number of phases and the neutral arrangement
- Fuel autonomy required, and whether a day tank is available
- Enclosure, acoustic limit, control panel and remote monitoring
- Certification required by the site: CE, ISO 8528 compliance, local grid codes
What to send for a quotation
With those nine answers we can propose a series, a model and a price, and show the derating calculation behind it.
Write to [email protected] or send the list through the contact form. If you already have a single-line diagram or a load schedule, send that instead — it answers most of the questions above at once.