In short. Generator capacity is the largest of three checks: the running load with margin, the load after the biggest start or step, and the load with one unit out of service where redundancy is required. This guide walks through each check and shows why one calculation is never enough.
Three capacity checks
| Check | Question |
|---|---|
| Running capacity | Can the generator carry the steady load with margin? |
| Transient capacity | Can it start the largest motor or take the biggest step within voltage and frequency limits? |
| Redundancy capacity | With one unit down, can the rest carry the critical load? |
Running capacity
Example (illustrative): demand 420 kW, power factor 0.85, growth 15 per cent, derating factor 0.95 for a hot site, target loading 0.8.
Required kVA = (420 ÷ 0.85) × 1.15 ÷ (0.95 × 0.8) = 494 × 1.15 ÷ 0.76 ≈ 747 kVA. A 750 kVA unit is the nearest, but is at its limit, so 800 kVA or a different arrangement might be considered. Check the actual derating from the manufacturer’s data. See derating.
Transient capacity
Check the largest motor start with other loads running, and the largest step. Voltage dip and frequency dip must stay within the limits that the loads and controls can tolerate. See motor-starting capacity and load-step screening.
Redundancy capacity
For N+1 designs, divide the critical load across units so that the remaining units carry it. For example, a 1,500 kVA critical load with N+1 using units of 750 kVA needs three units: two carry 1,500 kVA, one spare. See number of generators.
Choosing one large or several small units
- One large: lower cost per kVA, simpler, but a single point of failure and poor at light load.
- Several smaller: redundancy, better part-load efficiency, stepwise growth, but more complex and costly.
Things that change the answer
- Non-linear loads such as UPS and drives may require a larger alternator.
- Altitude and ambient temperature reduce engine power.
- Gas generators have different step capability than diesel.
- Long cable runs and voltage drop affect the delivered voltage.
Limits of a preliminary estimate
This is a screening method. Final capacity needs verified loads, the manufacturer’s performance data and often a transient study. It is better to find that an apparently adequate generator has a problem at the screening stage than at commissioning.
A capacity requirement in five numbers
| Number | Meaning |
|---|---|
| A. Running demand (kW) | Load to be carried continuously |
| B. Power factor | Converts to kVA |
| C. Largest motor start (kVA) | Peak step to survive |
| D. Derating (percent) | Site temperature and altitude loss |
| E. Growth margin (percent) | Planned expansion |
Worked example (illustrative)
A = 450 kW, B = 0.85, so 529 kVA running. D: 8 percent loss in summer heat, so required nameplate = 529 ÷ 0.92 = 575 kVA. E: 15 percent growth, 575 × 1.15 = 661 kVA. Choose a 750 kVA class unit if standard ratings are 625 and 750. Then verify the largest motor start and transient limits. If those fail, the answer changes: the starting check often forces a larger alternator.
Duty class
- Standby: generator runs only during outages; loading limit is higher.
- Prime: generator runs continuously with variable load; the maker’s prime rating applies, typically lower than standby for the same machine.
- Continuous: constant load.
Select the rating class with the maker’s definition. See standby power and prime power.
Next steps
Confirm with motor-starting guide and derating guide.
Common mistakes
- Sizing only on the running load.
- Applying growth twice.
- Choosing the size that just meets the requirement with no allowance for tolerances.
What to send us
Send your demand data, motor list, site conditions and redundancy target. Send your application details. See also kVA and kW sizing.
Questions engineers ask
What target loading should I use?
Many designs aim for about 70 to 80 per cent of rating at the design load, but the right figure depends on duty and the manufacturer’s guidance.
Do I size for the largest motor start?
You must check it. The size that handles the running load may fail at the biggest start.
How does redundancy change the size?
It adds units so that the critical load is carried with one unit out.
Is one big generator or several better?
It depends on redundancy needs, load profile and space. Several units add resilience and flexibility.
Figures are illustrative. This is a screening method, not a final design.