Sizing is a load question first
A generator is chosen to match what it will carry. Too small, and it may stall when a large motor starts, trip on overload or deliver poor voltage. Too large, and it costs more to buy and install, and engines that run lightly loaded for long periods can suffer. Getting this right begins with a careful picture of the load rather than a catalog browse.
This is a general explanation, not engineering advice. A qualified engineer or supplier should confirm any real sizing against manufacturer data and local codes.
Running demand
Start with a list of everything the generator will power at the same time. For each item, record the running power, usually shown on the nameplate in kilowatts, horsepower or amps and volts. Add them to find the total running load, but only count equipment that really operates together. A diversity assessment, meaning a sensible estimate of what runs concurrently, avoids oversizing without undercounting.
Two units of measure appear in generator ratings:
- kW (kilowatts) measure real power, the part that does useful work.
- kVA (kilovolt-amperes) measure apparent power, which is voltage times current without regard to how well they line up.
They are related by the power factor: kW equals kVA times power factor. Generator sets are commonly rated at a power factor of 0.8, so a hypothetical 100 kVA unit would supply about 80 kW. If your loads have a poor power factor, current is higher for the same real power, and the generator must supply it. Understanding power factor helps explain why kVA, not just kW, matters.
A simple hypothetical example
Suppose a small plant wants to back up these loads (all numbers invented for illustration):
| Load | Running power |
|---|---|
| Lighting | 8 kW |
| Compressor motor | 22 kW |
| Two pumps at 7.5 kW each | 15 kW |
| Controls and office | 5 kW |
| Total | 50 kW |
Adding a 20 percent allowance for growth and uncertainty gives 60 kW, because 50 times 1.2 is 60. At a power factor of 0.8, that corresponds to 60 divided by 0.8, or 75 kVA. This is only the running estimate. The next step is to check whether the unit can start the largest motor.
Starting demand
Motors draw much more current while accelerating than when running. For a simple across-the-line start of an induction motor, the starting current is often several times the running current. A generator must supply that surge without its voltage dropping so far that contactors drop out or electronics reset. The bigger and more frequent the motor starts, the more this matters.
Ways to manage starting demand include:
- Soft starters or variable speed drives, which ramp up voltage or frequency and reduce the surge.
- Staggered starting, in which a sequencing control starts big loads one at a time.
- Choosing a larger alternator to tolerate the dip.
- Using reduced-voltage starting methods on large machines.
Unusual loads also matter. Equipment with electronic power supplies or drives can distort the current waveform and may need a larger alternator or filtering. Compressors, crushers and loaded conveyors start under heavy torque and are demanding.
Because these effects are difficult to estimate by hand, suppliers commonly offer sizing software that models motor starts. Treat results as an input to a professional judgment.
Altitude, temperature and duty
Engines and alternators are rated under defined reference conditions. High altitude and high ambient temperature reduce available output because the air is thinner or cooling is poorer, so manufacturers publish derating guidance. Duty class also matters. A standby set is meant for emergency use, while a prime set can run for long periods. Choosing between them is related to the topics of standby generators and prime power generators, which have different assumptions about run hours.
Fuel type and runtime
Common fuels include diesel, natural gas and propane. The practical questions are:
- How long must the generator run in one stretch?
- Can fuel be stored on site, and do rules limit the amount?
- Is a pipeline supply dependable during the events that cut power?
- How quickly can more fuel be delivered?
Runtime is calculated from fuel capacity and consumption at the expected load. Manufacturers publish consumption at several load points, and consumption is typically not proportional to load, so do not assume half the load uses half the fuel. Keep in mind that estimates are not guarantees, and plan a margin.
Installation questions to ask early
Sizing is only part of the decision. Before ordering, clarify:
- Location. Indoors or outdoors, distance to the loads, and clearances for air, exhaust and service access.
- Exhaust and ventilation. Engines need large flows of cooling and combustion air and a safe exhaust path.
- Transfer arrangement. Automatic or manual transfer, and how the generator is isolated from the utility. This is not a do-it-yourself matter.
- Fuel storage. Tank size, containment and local rules.
- Noise and emissions. Local limits may require sound enclosures or specific engine certifications.
- Permits and inspection. Many jurisdictions require them.
- Grounding and protection. The generator’s neutral and grounding approach interacts with the facility’s grounding systems and must be designed correctly.
Common mistakes
- Sizing from the main breaker rating rather than the actual load
- Ignoring motor starting
- Forgetting future growth
- Overlooking derating at the real site conditions
- Leaving out maintenance access
- Assuming a bigger unit is always safer, when light loading can cause problems
Proper upkeep after installation, including generator maintenance on a schedule, keeps the unit ready to deliver what the sizing exercise promised.
Summary
Build a load list, convert to kW and kVA, check motor starting, apply site derating, choose a fuel that fits the desired runtime, and settle installation questions before purchase. Have a qualified professional verify the result.