If the generator is too small, it cannot carry the load during a grid outage; if it is too large, you pay more in capital and fuel costs, and the unit regularly runs underloaded. Below are the steps Dong Tam engineers typically follow to size a generator when advising plants.
1. Distinguish between kVA and kW
Generators are rated in kVA (apparent power), while loads are usually rated in kW (real power). The two are related by the power factor cosφ: kW = kVA × cosφ. For three-phase generators, manufacturers normally assume cosφ = 0.8. For example, a 500 kVA unit delivers a maximum of about 400 kW of real power.
2. List the loads to be supplied
Draw up a table of the loads that will run on the generator during a grid outage: motors, air compressors, pumps, lighting, air conditioning, office equipment… For each load, record the rated power, the quantity and whether it can run at the same time as others. Not all equipment needs backup power. Separating out the priority loads can significantly reduce the generator capacity you need to buy.
3. Account for motor starting current
Electric motors started direct-on-line draw a starting current many times their rated current, causing voltage and frequency dips on the generator. This is the most common reason why a generator that is “big enough” on paper cannot carry the actual load.
| Starting method | Starting current (indicative) |
|---|---|
| Direct-on-line (DOL) | approx. 6 – 7 × rated current |
| Star – delta | approx. 2 – 3 × rated current |
| Soft starter | approx. 2 – 4 ×, depending on settings |
| Variable speed drive (VSD) | approximately rated current |
Start large motors one after another rather than all at once. For high-power motors, consider a soft starter or a variable speed drive to reduce the required generator capacity.
4. Choose a Standby or Prime rating
Under ISO 8528-1, generators have two commonly used power ratings:
- Standby (ESP): used as a backup source during grid outages, with limited running hours per year and no overload allowed.
- Prime (PRP): used as the main power source with unlimited running hours and a variable load. The Prime rating is typically about 10% lower than the Standby rating.
If your plant has frequent power outages or uses the generator as its main power source, select it based on the Prime rating.
5. Add a safety margin
Once you have the total load and have checked the starting currents, add a margin of about 20 – 25% for future expansion. However, avoid oversizing: a diesel engine that runs for long periods below about 30% load is prone to carbon buildup and unburned fuel passing into the exhaust (wet stacking), which shortens engine life.
6. Other factors to consider
- Site conditions: high ambient temperature and high altitude reduce the engine’s actual output.
- The automatic transfer switch (ATS) and the required transfer time.
- Soundproof enclosure, installation location, exhaust system and ventilation.
- Fuel tank capacity based on the required continuous running hours.
Dong Tam supplies Inmesol generators in a wide power range, together with load survey, installation and maintenance services.
Need specific advice for your plant? Contact Dong Tam for support from our engineers.