How can you verify the necessary power for a generator?

How can you verify the necessary power for a generator?

There are several methods to calculate or verify the necessary power for a generator, learn the process and how to know which genset you need to purchase.

Mainly in Portugal at major projects, the designers have at their responsibility the calculation to verify the necessary power for a generator and then define it in the contract documents of the project.

In the case of private individuals, they are the ones responsible for this calculation, so the solution is to check the invoice of contracted power, although sometimes it is necessary to consider that the value in this invoice may not correspond to the reality, because some devices like the pumps have peak start-up. To reassure that the genset has the necessary power and due to these peak start-ups, usually we add +25% of contracted power. On the other side, we also need to be very careful to not overload the genset, because we are not going to use several devices at the same time, and a genset that works under 30% of its load, will bring problems to the engine.

If the consumed power is irregular or has many peaks (for example in the industry), we will have to use a device to take the measurements, and for this, we use the power analyzer. This process allows us to analyze the load of the installation for a week so that in the end we can extract the results of the equipment and see which power is necessary to supply the building. However, even after this analysis, it is necessary to verify other types of equipment that may not be working, but on other occasions can function daily, like for instance the air condition.

The process of installing the power analyzer is to insert it in the entrance cables of the main network and this needs to be done by a professional. This device reads the voltage and current measurements and calculates the active and reactive power consumed at every moment of the installation. At the same time, it analyses the harmonica of the voltage and current of the facilities, so that in this way we can see the feeding quality for our load, and this is a crucial fact to the genset selection.

In Portugal, the gensets work mainly in emergency systems and for that, it is important to verify the power needed in case of mains failure and only consider the equipment’s that are essential to the daily bases.

In case of doubt it is also possible to calculate the values of each device that you use daily and create a total of power needed and below we present a very simplified example of approximated powers (every case is different and need to observed/analyzed by a professional):

How can you verify the necessary power for a generator?

S(kVA) =  P(W) / (1000 × PF)


Genset of 12kVA


-However due to the simultaneity factor, because not all the equipments will be connected at the same time we suggest:



S(kVA) =  P(kW) / PF

Genset suggestion of 9kVA


Patricia Santos
Digital Marketing Assistant

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