Two generators displaying the same kW output are not necessarily intended for the same use. Check whether the rating is Standby or Prime Power.
Standby: backup power
A standby rating applies to backup use when the normal power supply is interrupted, such as a utility outage.
Permitted operating hours and average load depend on the exact classification and manufacturer conditions. Standby output should not be interpreted as power available continuously at full load.
Prime Power: primary power
Prime Power applies when a generator serves as the main electricity source with varying loads, for example at a construction site or a site without utility power.
It generally permits unlimited annual operating hours, subject to load limits, operating conditions and required maintenance. This does not mean unlimited operation at 100% of the stated output.
Why can one generator have two ratings?
A generator set may have a higher standby rating than prime rating. Compare models using the same duty rating. Do not apply a universal conversion percentage: use the specifications of the complete generator set.
A 20 kW example
Consider a generator specified at 20 kW Standby and 18 kW Prime Power.
- During an outage, its backup rating is 20 kW, subject to manufacturer limits.
- As the main power source, its reference rating is 18 kW, with varying loads and prescribed average-load limits.
A generator advertised as 20 kW does not necessarily provide 20 kW for prolonged primary use. Check the duty rating.
Illustrative example: these 20/18 kW values do not confirm the specifications of the Solution NRJ model.
Which rating fits your project?
Tell us whether the generator will provide outage backup or primary power, which equipment it will supply and the expected operating hours.
Sources
Cummins — T-030 application manual (PDF)
STAMFORD | AvK — Alternator and generator set ratings