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Differences Between Prime and Standby Generators

FW Power Guide

Differences Between Prime and Standby Generators

Firstly, before looking at the differences between Prime and Standby Generators, it is important to understand what these definitions mean. It is worth noting that there are more rating definitions than just Prime or Standby Power such as Continuous Power, Limited Time Running and Emergency Standby Power. Ratings for Prime and Standby Power are defined in ISO 8528 and ISO 3046 and whilst there may be small variances between the Generator manufacturers in their definitions they are generally as per the below:

Prime Power:

The maximum power available for a varying electrical load for unlimited hours. A 10% overload is available for 1 hour in 12.

Restrictions are often placed upon the average value of the varying load, typically 70% of the prime power and also on the overload operation which typically is for a maximum of 25 hours per year.

Where the load is not variable such as exporting to the Grid then the Prime Power definition is not used and Continuous Operation Power (COP) is used. If the number of hours at a non variable load are limited and not as high as COP then sometimes the rating of Limited Time Running (LTP) is used in place of COP.

Standby Power:

The maximum power available with a varying load for the duration of the interruption of the normal source power. No overload is available with a Standby rating.

Restrictions are generally placed upon the average load, typically 70% of the Standby Ratings and hours limitations are imposed of usually 500 hours per year but sometimes 200 hours per year.


A generator rated at 300 kVA Prime Power and 330 kVA Standby Power.

When used for Prime Power the generator can run at 300 kVA but the average load should be 70% of this rating i.e. 210 kVA. The number of annual run hours are not limited. For every 1 hour in 12 (subject to the manufacturers’ guidelines on hours limitations) the generator can run at a 10% overload giving 330 kVA.

When used for Standby Power the generator can run at 330 kVA but the average load should be 70% of this rating i.e. 231 kVA. There is no overload available for Standby Power and the manufacturers restrictions on run hours should be taken into account.

Uses of Prime and Standby Power:

Prime Power is the main (prime) source of power on the site, whereas Standby Power (emergency power) is to be used when the main source of power fails. The main application for Standby Generators is to act as emergency power for when the Grid (mains) suffers a power outage i.e. a power cut occurs.

Main Differences to Consider Between Prime and Standby Power:

Prime Powered Generators are run on a regular basis. As such fuel supply and maintenance are absolutely critical. Most generators have a base fuel tank which is designed for short run periods of typically 6-10 hours, so an externally connected fuel tank will be required to run the generator for longer periods. These tanks vary from 500 litres to upwards of 20,000L depending upon how long you wish to run the generator. Please view the FW Power Guide on Generator Fuel Consumption.

Maintenance intervals on most generators are 500 hours although they can be as low as 250 hours for certain engines. Thinking about maintenance at an early stage of the project and having a plan in mind can save both downtime and considerable expense in the future. Access to and around the generator is critical. Please view the FW Power Guide on Generator Maintenance.

Standby Generators are designed to run when the main provider of power – usually the Grid – fails. In order to transition the power from the failed supply to the generator a Transfer Switch is required. Ensuring that the generator interfaces with the transfer switch is critical and may require the installation and programming of a number of signal cables e.g. auto start signal, up to speed signal. Please view the FW Power Guide to Automatic Transfer Switches.

Fuel supply is also a consideration for Standby applications and the question should be asked – how long do I want my generator to run for in the event of an extended power failure? The answer to this question is often dependent upon how critical the supply is for either the integrity of your business e.g. a data centre or hospital or what level of financial disruption is caused by not having a generator running.

Because a Standby Generator is not run regularly to provide the main power it must be ran up periodically by the maintenance team to ensure that it starts and supplies the building load when required. A Standby Generator should be fitted with a trickle charge battery charger to keep the starting batteries ready for action and it is also advisable to fit a jacket water heater which keeps the engine at a pre-set temperature and significantly aids in cold starting, reduced engine wear and reduced smoking from the engine when cold.

We have not covered the sizing of your generator within this summary but Please View the FW Power Guide to Generator Sizing and Installation for more information.