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Factors Affecting Generator Output Rating

All electrical appliances have set conditions under which they function at optimum levels. Any fluctuations in these conditions can cause the appliances to run at a lower efficiency. Power generators are no exception to this. Generators are typically designed to run most efficiently at or near sea level under standard conditions of temperature and pressure (STP). Any fluctuation from STP conditions can impair generators and cause decreased output. Under extreme circumstances, generators can cease to function entirely.  For most applications, many of these factors are relatively minimal unless the generator set is run at elevations over 5000 feet above sea level, or has ambient temperatures that remain over 100 degrees Fahrenheit for a significant length of time.  Special care should be taken to compensate for these types of extremes and is discussed a bit further below.

Environmental Factors that Affect Generator Function

Ambient conditions of temperature are extremely crucial for proper ignition and functioning of a generator. All generators, irrespective of the fuel that powers them, require adequate air for combustion. Decreased air levels can lead to start-up failure. In diesel engines, air and fuel are infused together. The compressed air becomes hot and when peak temperature and pressure is achieved, diesel is injected, which then ignites under the given conditions. In generators using gasoline, an air and fuel mixture is introduced at once using a carburetor and a spark is induced to ignite the engine. However in both cases, adequate levels of air are required for proper start up and operation.

Altitude: In areas of high altitude, air pressure drops reducing the air density. This can create problems with generator start up if not accounted for since air is crucial for ignition in any type of generator. Another factor that gets affected is availability of ambient air to facilitate heat dissipation from the generator. A lot of heat is created during the combustion process and needs to be dissipated into the environment to reduce engine temperature. At high altitudes, due to the low air density, heat dissipation occurs at a much slower rate than it would at sea levels, resulting in high engine temperatures for a sustained period of time. The engine remains hot and overheating is a common problem in such cases.

Temperature: High temperatures are also associated with lower air density and can cause similar ignition problems due to inadequate air supply. This can burden the engine which pushes itself to deliver the power it is designed to. However, due to inadequate oxygen levels available for combustion, it fails to do so. In many such instances, the engine gets overheated and sometimes collapses altogether.

Humidity: Humidity is the measure of water content in a given volume of air. In conditions of extreme humidity, water vapor in the air displaces oxygen. Low oxygen levels impair ignition, since oxygen is the element in air that is ignited in an engine for the burning of fuel.

Rating Generators

Generators come in various sizes. Each of them is preset for particular output levels. Generators are selected and installed based on the power requirements of any facility. A typical generator is ideally set to run at 80% of its capacity for continuous usage. In an emergency, it may be utilized for 100% efficiency. Various companies manufacturing generators have now come up with standard ratings for these generators, which give the buyer an idea of actual generator capacity. As per the requirement of the consumer, he can then make a choice between available brands, since each brand capacity is standardized as per international standards.  Also view Sizing a Generator for further information in determining what generator capacity is proper for different needs and situations when using standby power is necessary. 

Derating Generators

We have seen above how non-standard environment conditions can reduce generator power output. In such cases, how does one guess the new output levels? A technique called 'Derating' is employed to determine the generator’s performance under new ambient conditions. Derating is defined by Wikipedia as “the technique employed in power electrical and electronic devices wherein the devices are operated at less than their rated maximum power dissipation”.

Derating a generator depends on the manufacturer of the unit. Different manufacturers design generators using materials from different sources. Also, design developments are non-similar and so is the technique in many cases. All these can contribute to overall generator efficiency. Therefore, derating a generator is dependant on the manufacturing process. Different brands have different derating factors for estimating generator output under non-standard ambient conditions. 

However, a general formula can be used to calculate close estimates for the output levels. The standard derating formula states that for every 1000 ft above sea-level, a gasoline, diesel, or liquid propane generator usually should be derated by 2–3% of its standard output. In case of generators using natural gas, the derating factor is typically closer to 5%. 

Fuel Problems

At low temperatures, along with inadequate oxygen levels, which cause problems in start-up, another common problem that occurs is the gelling of diesel fuel. Low temperatures cause diesel to gel, altering the flow characteristics of the fuel. This gelling is attributed to the paraffin content in diesel. Some diesel types, such as low sulfur diesel, have higher paraffin content than others. At low temperatures, paraffin crystallizes and clogs the fuel filters. When the fuel filters get clogged, additional fuel cannot enter the combustion chamber with ease and the air to fuel ratio changes leading to inadequate combustion. In such conditions the generator engine may fail to start.  Also visit Generator Fuel Tanks to learn further about the care and storage of diesel fuel.

In order to avoid gelling, two methods are generally used: a) winterizing of fuels and b) adding anti-gelling additives to the fuel.

Winterizing is a process where commercial grade fuel is mixed with more refined fuel in predetermined ratios to decrease the overall paraffin content of the fuel. This is generally done at distributing agencies before the fuel is delivered to gas stations. Different geographic regions have different mixing ratios depending on temperature conditions. In an area of very low temperature, or in a case where the diesel fuel has more paraffin content, higher levels of refined diesel are present in the mixture.

Anti-gelling agents prevent the gelling of diesel. They alter the chemical qualities of the fuel so as to prevent paraffin crystallization and prevent diesel from gelling. It is recommended that the anti-gelling agents be added to the fuel tank before filling up the tank. These additives also have to be mixed properly in the stated ratios. If your fuel filters are already clogged, variants of anti-gelling additives are available that de-clog the filters and prevent further clogging.

Load Considerations

As discussed in the previous sections, it is understood that STP conditions enhance generator power output due to maximum air availability and desired flow quality of the fuel. Although designed to bear 100% load during standard conditions, it is typically advised that generators be run around 80% of their total capacity for maximum and continuous usage. In times of emergencies, however, the generator can be pushed to deliver 100% output for critical circuits. From a maintenance point of view, this does not overload a generator and the life span of the genset is not affected negatively.  


In totality, it is best to use generators with the specifications they come with from the manufacturer at conditions of STP. In non-standard conditions, it is advised to derate the generator as per the manufacturer’s derating formula and operate the unit accordingly so as to avoid overburdening the generator. Fuel must be winterized or mixed with anti-gelling agents in low temperatures. Regular maintenance and repairs must be taken care of to enhance the life span of the generator since a well maintained generator is a valuable investment that can save you from precious losses in times of power failure.  As is the case with any complex electrical equipment (like an industrial generator set), a trained technician or experienced electrical contractor should always be consulted prior to attempting any type of modications, not only for safety and reliability, but to ensure you are getting the best possible performance from your equipment.

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