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How to Choose Power of Diesel Generator Set

Mar. 01, 2019

When we purchase and select diesel generator sets, they often confuse the relationship and definition between apparent power, active power, rated power, maximum power and economic power. For this reason, we have sorted out the relationship between them so that everyone can understanding.

How to Choose The Power of Diesel Generator Set.jpg


1. The unit of apparent power is KVA, which is used to express the capacity of transformers and UPS.


2. The active power is 0.8 times the apparent power, and the unit is KW. It is used to power generation equipment and electrical equipment in China.


3. The rated power of a diesel generator set is the power that can be continuously operated for 12 hours.


4. The maximum power is 1.1 times the rated power, but only 1 hour is allowed within 12 hours.


5. The economic power is 0.75 times of the rated power, which is the output power of the diesel generator set that can be operated for a long time without time limit. When this power is running, the fuel is the most economical and the failure rate is the lowest.


When selecting a diesel generator set, you can measure the amount of load you need beforehand, and then purchase the generator set according to the load.


1. First of all, it is necessary to distinguish the difference between the apparent power and the effective power of the diesel generator set. The relationship between them is 1KVA=0.8KW. The imported genset generally uses KVA to represent the power generation, while the domestic power equipment is generally represented by KW, so when calculating the power, the KVA should be converted into KW by 20%.


2. Secondly, it is necessary to determine the rated power and maximum power of the diesel generator set to be selected.


3. In addition, determining the power of the generator set does not simply add up the power of all the appliances, but also needs to consider the starting mode of the appliance. The general starting mode is divided into direct starting and step-down starting or variable frequency starting. The former starting current is about 6 times of working current, the instantaneous power will reach 6 times of rated power. The latter starting current is about 3 times, and the instantaneous power is also power 3 times. Therefore, when selecting the power of the generator set, in addition to considering the sum of the power of all the appliances, it is also necessary to consider the starting power of each appliance.


Why Size of Diesel Generator Should I Purchase?


Under normal circumstances, the diesel engine matched with the generator users 12h power or continuous power as the calibration power. When the 12h power is used to indicate the calibration power, it indicates that the diesel engine has a continuous running time of 12h under standard power (standard environmental conditions), including continuous operation for 1h under more than 10% of the rated power, when continuous power is selected as the calibration power, it means the diesel engine allows for long-term continuous operation, including up to 10% of rated power for 1h. Typically the continuous power is 90% of the 12h power.


The calibration power indicated on the nameplate of the diesel engine is determined according to the specified standard environment. When the environmental conditions and the standard regulations are different, the power shall be corrected according to the above method. When supporting, the diesel engine should have sufficient power to ensure that the generator outputs the rated power under the condition of calibration operation.


According to experience, the ratio of diesel engine power to generator power is 1.35:1 for plain fixed generators and 1.6 for mobile generators. Table 1 below shows the matching characteristics of various diesel generator sets.


Table 1 Matching characteristics of various diesel generator sets



Mobile Genset

Fixed Genset

Marine Genset

Matching Capacity










Complete Set Type

Generator set (including diesel engine, click control box, etc.), trailer type, car type (train type)

Fixed Installation

Fixed Installation


Mobile Communication and Mobile Backup Power


Basic power supply in remote areas and areas where electricity is tight. Backup power for important departments

Marine auxiliary machine control and lighting power supply


Continuous working time / h




Rate adjustment

Instantaneous rate/%






Steady state rate /%





Speed fluctuation rate /%





Working condition

Temperature/ ℃










Altitude / m






Inside the cabin, open air = tunnel





Matching power



12h or Continuous Power


Continuous Power


Matching ratio






Load Characteristic


① Constant speed, variable load continuous operation, short-term overload.

② Fast start and load operation (10-18s)

③ With impact load, such as sudden increase of 100%, short-term over-current of 50% ~ 100% short-circuit, start-up asynchronous motor




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