How can you verify the necessary power for a generator?

There are several methods for calculating or checking the required power for a generator.

In Portugal, in major works, the designers generally have the responsibility of carrying out the calculation of the required power for a generator and establishing it in the works specifications.

When this responsibility rests with private individuals, the solution is to consult their electricity bill to check the contracted power. It is necessary, however, to consider that the value in the electricity bill may not be the amount used, as there are some devices, such as pumps, that have peaks.

To ensure that the generator will have the necessary power, in the case of these peaks, normally, 25% is added to the contracted power. On the other hand, we also must be careful not to oversize, as a generator that works at less than 30% of the load may have engine problems.

In case the consumed power is irregular or has many peaks (frequent in industries), we use the electricity analyzer to make the measurements. This process allows us to analyze the load of the installation for a week so that, in the end, it is possible to determine the power required to power the building. However, even after this analysis, it is necessary to check other types of equipment that may not be working, but whose use may be daily, at other times, such as the air conditioning.

The electrical energy analyzer is placed in the cables at the grid entrance, which must always be performed by a professional. This device measures voltage and current, and calculates the active and reactive power consumed, at each moment, by the installation. At the same time, it performs harmonic analysis of the voltage and current in the installation, in order to determine the quality of the supply for our load, which is an important factor in the selection of the generator.

In Europe, generators operate mostly as emergency systems, therefore, it is important to check what power is required in the event of a network failure, considering only the essential equipment of daily life.

When in doubt, it is also possible to calculate the values of each device, individually, to create a total of necessary power.

Below is a very simplified example of approximate powers (all cases are different and must be observed with the help of professionals):


S(kVA) = P(W) / (1000 × PF)


Calculation of 12kVA generator

However, due to the simultaneity factor (because the equipment will not all be connected at the same time) we suggest:



S(kVA) = P(kW) / PF

Suggested generator 9kVA

Grupos electrogéneos Grupel+Grupel - Energia de Portugal para o Mundo