How to calculate the generator requirement you need

Technicians analyzing blueprints and details of an electrical project in an industrial setting.

Selecting the right genset is a critical task that goes beyond simply estimating power. An incorrect calculation can result in oversized equipment, which implies unnecessary costs, or undersized equipment, compromising the operation of the entire installation.

Below, we show you how to correctly calculate the generator set you need, considering all the technical factors involved.

How to proceed with power calculation

Firstly, you need to determine the required power, carrying out a load assessment. In order to do this correctly, it is also necessary to consider the start-up peaks caused by some types of loads such as electric motors.

Load assessment: The first step is to identify all the loads that the genset will have to feed. This includes machinery, electrical systems, safety equipment, and any other critical devices to be powered by the genset. It is essential to add up the constant powers of all these loads considering that all the loads might not be connected at the same time.

Consideration of start-up peaks: Some machinery has a peak electrical demand at start-up which can be between 2 and 5 times higher than their consumption in normal operation. Most commonly, these are the ones driven by electric motors, some examples and their classification are given below:

Light starting: Turbines and fans (2 to 3 times of normal consumption).
Medium start: Conveyor belts and compressors (3 to 4 times of normal consumption).
Heavy start-up: Cranes and lifting equipment (4 to 5 times of normal consumption).

Technicians working on the maintenance and inspection of a generator in an industrial plant.

Calculation will also depend on the type of motor drive. If the electric motor is powered by a frequency inverter (or other advanced system) the starting peak may vary. E.g. Direct starts, the most unfavourable case.

It is also important to know the moment each load shall be started as it may be the case that all the loads do not start at the same time, this is called the load-step start.

F.i. In order for the diesel engine to be able to handle the high starting peaks, a genset of twice as much power as the highest starting peak is considered.

Total power calculation: Sum of the constant powers identified in the load assessment.

Table for calculating the power of a generating set

The above calculations can be expressed in a table like this one:

Equipment / Load	Constant Power (KW)	Simultaneity factor	Starting factor	Start-up Power (kW)	Load-Step Start	Total Power (kW)
Machine A	5	1	3	15	1
Machine B	8	1	2	16	1
Lighting	3	1	1	3	2
Compressor	10	1	4	40	2
Fans	4	1	2	8	1
Total Constant Power	30 kW
Load-Step 1 Start-up				39 kW
Load-Step 2 Start-up				43 kW
Needed Power						86 kW (2 x 43kW)
Safety Margin (10%)						94.6 kW
Power Factor (0.8)						118.25 kVA

How to use this table

Equipment or load: List the equipment or loads to be connected to the genset.
Constant power (kW): Enter the rated power of each piece of equipment in kilowatts (kW).
Simultaneity factor: Indicate a figure to express how many loads are operating simultaneously.
Start-up factor: Apply a start-up factor for each equipment according to its type (e.g., 2 for light start-up, 4 for heavy start-up).
Starting power (kW): Multiply the power by the starting factor.
Starting step: Indicate the different steps to express which loads start simultaneously.
Total power (kW): Add the starting powers to obtain the total power required.
Safety margin: Apply a safety margin (10% in this case).
Power Factor: Divide the adjusted total power by the power factor (normally 0.8) to obtain the apparent power in kVA, which is used to select the genset.

Generator calculation formula

kVA = (Total Power in kW x Safety Margin) / Power Factor

The generator calculation formula allows technicians to accurately size a generator set based on real power demand and environmental conditions. The basic formula involves adding the constant load power (kW), considering start-up peaks, applying a safety margin (typically 10%), and dividing by the power factor (commonly 0.8) to convert kW to kVA. This ensures the selected genset can handle both steady loads and starting surges efficiently. Adjustments must also be made for altitude and temperature when applicable.

Snowy mountain landscape illustrating the challenges of operating generators in cold climates, where low temperatures can affect engine startup and efficiency.

Environmental conditions affecting genset power requirements

Extreme environmental conditions can have a significant impact on the performance and efficiency of a generator set. It is crucial to consider these variables when calculating the required power and selecting the right equipment.

Extreme temperatures

Low temperatures: In cold climates, engine start may be slower and engine oil may thicken, reducing efficiency and increasing wear. It is essential to consider a genset with engine & fuel preheating systems, as well as oil suitable for low temperatures.
High temperatures: Excessive heat can cause the engine to overheat and reduce the cooling capacity of the system. Generator sets in these environments should be equipped with enhanced cooling systems, such as larger capacity radiators or additional fans.

Altitude

At higher altitudes, air density decreases, which affects both combustion and cooling capacity. This results in a reduction of the power available from the genset. It is considered that for every 300 metres of altitude above sea level, engine power decreases by approximately 3-5%.

Adjustments required according to environmental conditions

Power adjustment: Recalculate the required genset power to consider losses associated with altitude and temperature.
Selection of suitable components: Ensure that the genset has specific components for operating in extreme environmental conditions, such as enhanced cooling systems or corrosion protection.
Additional maintenance and testing: Implement a regular maintenance programme that includes FATs in the actual environmental conditions in which the equipment will operate to ensure optimal performance and prevent unexpected failures.

Taking these environmental factors into account is essential to ensure that the genset will operate reliably and efficiently, regardless of the conditions it is exposed to.

Genesal Energy Engineering Department takes all these critical factors into account when designing and selecting the most suitable generator set for each project. We ensure that each piece of equipment is perfectly adapted to the specific environmental conditions and energy needs of our customers, guaranteeing optimum performance, durability and efficiency, no matter where.