Steps to choose the right generating set

Pasos para seleccionar un grupo electrógeno

Correctly sizing a generator is a key factor in avoiding the loss of efficiency and durability.

Each energy-related project supposes a different challenge and different requirements one must comply with, and electrical dimensioning is increasingly complex

In the past, dimensioning calculations were based on the arithmetic sum of power ratings together with some correction factors of the power factor according to the number of engines involved in the installation. The type of load was simpler, more close to being purely resistive, or there would be some types of load which could be just that. Today such a thing is unthinkable, the electrical devices which connect to an installation have a very high load of electronic components; the very factor which facilitates the use of such devices causes the generation of energy to be more complex, with the appearance of harmonics and inductive and capacitive loads.

It is for this reason that GENESAL ENERGY’s sales representatives receive continuous technical training and are closely supported by the engineering department in order to offer the clients an optimal solution.

When doing an analysis focused on power rating, this analysis can be centred on two circumstances: choosing a under-dimensioned or an over-dimensioned electric generator.

  • If it is under-dimensioned: the main problem from the end-user’s point of view will be the insufficient power, the generator set would be required to provide power for which it was not designed, which could lead to a drop in voltage, a drop in the velocity of the engine, which, in turn, would lead to the alternator suffering a drop in frequency and over-heating, due to the fact that the alternator’s output current far exceeds its dimensions and could affect the coating of the isolators from an electrical point of view.From a mechanical point of view, it would cause an increase in the engine’s working temperature, as well as an increase in the wear of the internal components and vibrations; one must remember that the optimal working point of the engine is between approximately 70% and 80%.
  • If it is over-dimensioned: in order to understand the problem of over-dimensioning, we must have recourse to a comparison, the engine of a genset is working in the same conditions as those of a car climbing a slope. If we over-dimension the engine of a generator, it would also work outside the 70%-80% range which we indicated above as its point of optimal functionality.The main problem in this case is a mechanical one, which leads to an array of other problems, from working beyond the range and below the optimum load threshold, there would be a drop in the working temperature, which would lead to poorer combustion, thereby affecting the induction, as a well as the anti-contamination systems such as filters and valves, causing fumes in the exhaust, ejecting suspended and unburned particles.

What are the keys for reaching the appropriate decision?

Finding just the right measurement

The initial and most basic step is to define which devices will be connected to the power generator and to know the necessary power rating for the project. In other words, to know the total power of all the electrical devices which will need to be powered by the genset, applying simultaneity rates wherever possible and always responding to the client’s requirements and needs.

However, a simple mathematical operation will not give us enough information. There are other factors which much be taken into account, for they affect the result of this calculation. They have to do with technical aspects, as well as with usage and environmental ones.

It is necessary to have a more thorough control over the greater loads in the installation; electric motors, illumination for discharge lamps or LEDs, are all electrical loads which are difficult to power because they involve transitional processes which can cause peak consumptions lasting for split-seconds, which badly designed devices will not be able to sustain.

Sizing the generator set

In order to size the genset and decide on its configuration, the team of engineers will take as a basis the electrical, mechanical or civil engineering project. This will define:

  • The necessary power rating and load types required in order to cover present and future needs.
  • The elements which the device needs to have (manual or automatic start, operating in parallel with other devices or with the public mains, sound insulation, auxiliary fuel storage, heaters, etc.)
  • The legal standards one has to comply with (electricity, noise, gas and particle emissions, transportation norms in the case of mobile generators).
  • The location where the device(s) will be installed (under a roof, outdoors, elevation above sea level, dusty environment, etc.)

Once GENESAL ENERGY’s group of engineers receives this information, they will begin to devise their project, in order to comply with your needs, thereby adapting it not only to the technical requirements, but also to the actual use that the genset is meant for.

Application and frequency of use

Some uses may require safety margins with regard to power rating, it is not the same thing to supply continuous power in order to sustain a constant electrical load without interruption (COP generators) or to do the same but with variable power (PRP-STP generators) or to respond to emergency or standby cases (ESP generators). For this reason, it is necessary to correctly define the energy needs which will be demanded from the generator, in order to provide a solution tailored to your needs.

Environmental conditions

The location where the genset is installed is a decisive factor. We at GENESAL ENERGY can provide solutions for:

  • Saline environments.
  • Marine generators.
  • Mines and dusty environments (IP 45).
  • Nuclear zones.
  • Earthquake-proof electrical panel.
  • Extreme temperatures (-32°c to +50°c).
  • Mobile generators reinforced for military use.

Elevation of the installation

This factor may come as a surprise, but if it is not taken into account, it may destroy the engineering work entirely. The engines need oxygen in order to function and the oxygen concentration decreases as the genset’s location’s altitude increases. This leads to the mixes inside the combustion chamber being poorer in oxygen, thereby decreasing the engine’s power output by up to 10% in the first 1000 metres of altitude if the engines have atmospheric aspiration. However, in the case of turbo engines, there will be no loss of energy output at this level.

The type of load we must power

Another key step in sizing the required power in a genset is knowing the type of loads it will be powering. One must take into account that some devices (pumps or engines using convertors) are subject to transitory power surges during the starting process.

Special attention must be given to the following loads:

  • Uninterruptable Power Supplies (UPS). The nominal power of an alternator is over-dimensioned, depending upon the installation.
  • High engine loads. They can increase the starting power by up to six times, depending on the type of start-up.
  • Illumination loads. Especially illumination with sodium vapour and metal halides and, nowadays, LED illumination, due to the amount of transients generated by the circuitry that composes them.
  • Variable frequency devices (VFD). They can cause an over-dimensioning of 40 to 100%.
  • Loads particularly sensitive to transients, including powerful electronics, or which – like a computer – could have many electronic components.
  • Loads which require voltage and frequency variations for their operation.

Start-up current needed to counter a power outage

This will vary according to the motor’s start type. The general rule is:

  • Electric motors with star-delta start: the “start-up current” equal to 3 times the nominal or operating current.
  • Direct starting motors: demand 6 times the nominal current.
  • Starter motors with variable frequency drives: one must pay special attention to its data sheet, because the ramp and speed increase is normally configurable.

Necessary emergency power

This is yet another crucial factor: in the case of an emergency, the power necessary for the simultaneous start of various devices will be higher than the one needed for the sequential start seen in a scenario of correct operation (over 30 % more).

In order to calculate this standby or emergency power, we shall apply a power factor of 0.8 to the calculated maximum power.

Areas which must continue with basic illumination

In order to determine this value, we shall multiply the total number of light sources by the consumption of each one of them.

Once all these factors have been assessed, we will have an estimate of the size of the generator we need to install.

As we have seen, sizing a genset is an intricate process.

Therefore, it is appropriate that it be performed by experts. Genesal Energy has more than 20 years of experience in the energy distribution sector. And we continue to innovate, in order to provide our customers with personalized solutions of utmost efficiency.

Example: How to estimate the power of a generator set?

We will conclude with an example on how to assess the power of a generator for a 12-storey apartment building, which needs to support the following equipment:

  • 1 15 kW elevator: start with variable frequency drive (VFD), 380 V.
  • 2 water pumps: 10 HP each, star-delta start (S-T), 380 V.
  • 1 boiler pump: 5 HP, direct start (DOL), 380 V.
  • 1 pressurized stairwell for fire emergencies, 15 HP, direct start (S-T), 380 V.
  • Common hallways illumination: 5.5 kW (55 light sources of 100 W each), 220 V.
  • Entrance hall illumination: 1.5 kW (15 light sources of 100 W each), 220 V.
  • Vehicle entrance gate: 0.5 kW, direct start (DOL), 220 V.

We will then proceed to devise the load table and determine the starting power for each device. In this case, we will consider the scenario including the highest possible demand for calculating the genset: one with an elevator and another with a pressurized stairwell, given that both cannot function simultaneously.


Now we have given you this example, but each project is unique. Request our load chart in the form below if you want more information about choosing the right power for a generator set.