Standby or Prime Generator Power Explained

In this power continuity knowledge base article, we will explain the differences and advantages between standby and prime generator power.

Standby Generator

Standby Diesel Generator means Emergency Power for use.
When the main GRID supply fails during a power outage and your installed ‘Standby Generator’ starts up to supply power to your building.
Emergency Power generator is the same as the term ‘Standby’ generator.
Standby means that the generator will run for no more than 200 hours in any 12 month period.

Important that the load being supported by the standby generator is not greater than the maximum size amps that the generator can supply.

Remember that a ‘spike’ load can require more than the output of your standby generator. Sizing is all important. Together with the fact that any generator can only accept 65-70% of its maximum capacity in the first hit upon start up.
This applies whether your system is a Manual Start/Stop or fully automatic installed system.

Standby Generator Fuel Tanks

Another point is to ensure that the diesel fuel tank capacity is sufficient for any emergency standby power situation. In the event of power failure, you may be dependent on your standby generator for more than 10 hours. Hence can the capacity of your diesel fuel tank and also ensure that the tank is topped up.

Here is an example video of contaminated fuel before fuel polishing:

Diesel Fuel standing 12-24 months, needs fuel polishing to remove microbial build up. Fuel Polishing before and after
Microbial build up clogs up the filters and the injections and is a real problem for all diesel standby generators.
Check the fuel and test regularly.
Also ensure that the battery charger and engine block heater are always connected to the auxiliary supply. Standby power isn’t out of mind, out of sight. Don’t turn off.

NOTE:- Standby rated generator allows for a maximum of one hour above prime rated capacity in any 12 hours. Do NOT try to run at maximum capacity for long periods on any Standby Generator system. For continuous running you require a Prime Power rated generator.

Prime power over Standby, what is the difference

It can be quite a challenge for potential generator buyers to compare “apples with apples”.
You might find a generator that is a 100kVA prime power. Also one that is 110kVA standby power rated. You might think that the second one is a larger generator.
In fact, they are actually the same power rating.

Prime Power

Prime powered generators are effectively able to run for an unlimited amount of time at variable loads up to the maximum rated power. This also includes a 10% overload capacity for a limited period (1 hour in every 12 hours).

Continuous Power

Continuous power is similar to prime power but it has a baseload rating.PowerContinuity-standby-generator-power
This is the rating at which it can supply power continuously, but it is not able to handle overload situations or work well with varying loads. Therefore a continuous power generator is designed to provide power for a constant load. They are not designed to support varying or random loads. Unlike a Prime Power Generator, a continuous power generator doesn’t have any spare head room.
Double check all the loadings before choosing a continuous power generator.

Remember, Standby power generators are for emergency power use.
Expected to only run during a power cut. Over a year, you wouldn’t expect a standby generator to run more than 200 hours.
For this low run time, the generator is serviced twice a year and regularly tested.

Prime Power and Continuous Power Generators require much more servicing than Standby Generators. In fact, they will require a full service every 450 hours of running time. Never scrimp on servicing. Regular service and maintenance will ensure your generator will last for years of faithful service.

Prime Power and Continuous Power Generators will run for years but at around 13-14,000 hours runtime, they will require a re-build.

When choosing a Prime Power Generator ensure you know the maximum amps required by your equipment together with any possible spike loads. Then add 25% above that figure to allow head room for both today and the future uses.


Prime Power generators should not be run at below 50% of its rated prime load.
Low loading will cause the engine to not burn fuel properly and lead to both black smoke emissions as well as ‘wet stacking’.
20000_litre Diesel Generator tanks
This build-up of unburnt fuel and soot in the exhaust is termed, ‘wet stacking’. Un-checked it will damage the engine as well as decrease efficiency by using unnecessary amounts of fuel.

Standby & Prime Power Fuel Tanks

Diesel Fuel tanks are double skinned and bunded.
Standby Generators have a fuel tank which starts from 5-10 hours run time capacity.
Prime Power Generators will use a lot of diesel fuel therefore they will have a separate external fuel tank that may be anything from 2,000 litres up to 20,000 litres.
Always keep topped to avoid the generator sucking up any debris that may be at the bottom of the tank.

NOTE:- NEVER allow a diesel generator to run out of diesel fuel

THINGS TO CHECK (Standby & Prime Power Generators )

  • NEVER use CHEAP diesel or recycled diesel fuel.
    This will clog the filters and damage the injectors.
  • NEVER leave the fuel cap off as this allows contaminates to enter.
  • NEVER allow a diesel generator to run out of diesel.
    Bleeding the system takes time and is expensive. Plus you can damage the engine beyond commercial repair.
  • NEVER allow the generator to be using fuel from the bottom of the tank
  • Exhaust
    Ensure the flap on the exhaust is operational.
    IT is down/closed when the engine isn’t running and opens easily/lifts when the generator is running.
  • Rust on the silencer and repair before holes develop
  • Check around the engine and control panels for any rodent action.
  • Coolant is full and clean.
  • ALWAYS ensure the generator is supporting a suitable load for its size.
  • NEVER allow the generator to run with low or not existent loads for extended periods


Here is another article you might be interested in – HOW TO SIZE A STANDBY & PRIME POWER GENERATOR

Please check our FREE power calculator app

Either from the App store or from Google Play store.

Remember that any generator can only accept approximately 65-705 of its capapcity in the first hit from startup to supporting the load. Otherwise the generator will stall and stop. For more useful information, please check out the knowledge base on this website.



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