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How to calculate the power I should contract for my home


One of the most important aspects when making an electricity contract is the correct selection of the power to be contracted in the property, so in this entry, we will define the concept of power, we will see the options available for calculating the demand, we will discuss the types of existing power, as well as the installations, the factors that determine the optimum power to contract and how to request a change in the contracted power.

What is the definition of light output?

The contracted electrical power is the total amount of energy that a property can consume at a given time, measured in kW.

For example, the power contracted with Endesa entails a fixed cost of between 0.0036 euros per kilowatt per day and 0.0944 euros per kilowatt per day. Therefore, a correct selection of the power required in a home will lead to monthly savings on the bill without causing problems with power cuts.

However, this power may be modified, but this must be done at least one year after the conclusion of the contract.

How can the required power be calculated?

In order to be able to make a correct calculation of the power that will be necessary in the building, there are different alternatives:

  • Contact the company responsible for the commercialisation of electricity and ask that an approved technician carry out the energy study.
  • Use one of the power calculators available on the internet, such as the OCU, for example.
  • Makemanual estimates, taking into account the number of electronic appliances in the home, their individual consumption and their possible simultaneous use.

Although the three proposed options can be effective, Selectra prioritises the use of an expert to carry out the energy demand calculations.

Are there different types of power?

There are two standardised types of power supplies, depending on the type of installation this is the contracted power, the following table shows the distinctions between installations.

ICP current in amperes

Kilowatts in installation


Kilowatts in installation


5,0 A

3.464 kW

1.15 kW

7,5 A

5.196 kW

1.725 kW

10 A

6.928 kW

2.3 kW

15 A

10.392 kW

3.45 kW

20 A

13.856 kW

4.6 kW

25 A

17.321 kW

5.75 kW

30 A

20.785 kW

6.9 kW

35 A

24.249 kW

8.05 kW

40 A

27.713 kW

9.2 kW

45 A

31.177 kW

10.35 kW

50 A

34.641 kW

11.5 kW

63 A

43,648 kW

14.49 kW

The main difference between a single-phase installation and a three-phase installation is that a single-phase installation has only one circuit to supply the entire building; however, in three-phase installations there are three circuits distributed throughout the house, in three-phase installations larger amounts of power are needed so that the power control switch does not trip due to overloads.

How do I know what type of electrical installation I have in my home?

The type of installation is reflected in the Electrical Installations Bulletin. The most common type of installation in our country is a single-phase installation, as three-phase installations are designed for larger installations, although some homes have three-phase installations.

How do I know which electrical power to contract?

Among the factors that determine the energy needs of a dwelling are the following:

People who regularly occupy the household, the more people, the higher the energy demand.

Appliances used, as the total number of appliances and their possible simultaneous use will determine the energy needs.

Square metres of the dwelling, the more square metres, the higher the energy demand of the building.

➔ The presence of air conditioning installation, as these are machines that require a large amount of energy for their use.

➔ The presence of heating, as long as it is a case of electric heating, implies very high energy demands.

How can I change the power of my contract?

In order to change the contracted power in a property, a series of documents must be provided, as well as having waited a year after the last power change, contacting the energy supplier and requesting the power change, providing in this case.

  • Provide the ID card and contact details of the service holder.
  • Indicate the power currently contracted and the power to be contracted (always select values that the installation can withstand).
  • The CUPS code; this is a numerical code that is unique to each installation.
  • Account number for the direct debit of the charges associated with the contract.


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