A generator’s headline number is not enough to size a system. You need to know whether the rating is kW or kVA, whether it is continuous, prime or standby output, and what fuel, voltage, phase, altitude and temperature the figure assumes.
Use the manufacturer’s data sheet for the exact model. Similar-looking generators can have different load-factor limits, overload capability and output on diesel, natural gas or propane.
kW, kVA and power factor
Kilowatts, or kW, measure real power available to do useful work. Kilovolt-amperes, or kVA, measure apparent power, which includes the effect of current that does not produce useful work in an AC load.
Power factor = kW ÷ kVA. A generator rated 100 kVA at 0.8 power factor supplies 80 kW at that rating. Do not assume every generator or load uses 0.8.
For a single-phase system, approximate current is:
Amps = kVA × 1,000 ÷ volts
For a three-phase system:
Amps = kVA × 1,000 ÷ (1.732 × volts)
Standby, prime and continuous ratings
- Emergency standby: Backup power for outages, with limits on annual hours, average load and overload capability set by the manufacturer.
- Prime: Power for a varying load over extended or unlimited annual hours within the published load-factor and maintenance limits.
- Continuous: Power for a relatively constant load for unlimited hours at the stated rating.
- Limited-time running: High output for a restricted number of hours in defined applications.
These categories are influenced by ISO 8528-1, but manufacturers apply their own conservative definitions. A standby rating must not be treated as continuous output.
Running watts and starting watts
Portable generators often advertise both. Running watts are the output available continuously under the published conditions. Starting watts are a short burst for motors and compressors.
Size from total running load plus the largest additional starting demand expected at one time. Also check the limit of each receptacle, because one outlet can overload before the whole generator reaches its rating.
What can reduce usable output?
- Natural gas instead of propane on a dual-fuel standby model
- High altitude and high ambient temperature
- Low fuel pressure or undersized pipe
- Nonlinear loads and poor power factor
- Large motor-starting voltage dip
- Unbalanced single-phase loads on a three-phase set
- Frequency or voltage configuration changes
Never convert a 60 Hz generator to 50 Hz by changing engine speed alone unless the manufacturer provides an approved configuration. Voltage, cooling, alternator design and connected equipment must also be considered.
Generator sizing checklist
- Required voltage, phase and frequency
- Total real power in kW
- Load kVA and power factor
- Largest motor or transformer starting requirement
- Duty rating and expected annual hours
- Fuel type and site derating
- Harmonics, load steps and allowable voltage dip
- Future loads and load-management strategy
For a home standby system, have the installer complete both an electrical load calculation and a full-load fuel assessment. Commercial and industrial sets should be selected with manufacturer sizing software using the actual load profile.
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