As of 2010, from Directive 2010/31/EU of the European Parliament and the Council of 19 May 2010 on the energy performance of buildings, and other related directives, it was established that the Member States of the European Union should implement regulations to promote energy efficiency certifications in buildings.

In Spain, the CE3X energy simulation program can be used to meet this requirement, however, the program uses a simplified simulation method that may not be accurate. In Sweden, on the other hand, the program IDA Indoor Climate and Energy (IDA-ICE) is widely used. Will it be possible to obtain similar results if we use both programs to simulate the same building?

In the thesis, for the above reasons, the energy behaviour of the same building is studied with these two different programs. IDA-ICE, which uses a detailed simulation method, and the CE3X software, which uses a simplified simulation method. Also, the operation of the program is explained in detail.

In addition to simulating the building, different energy efficiency measures are studied, and the results obtained between the two programmes are compared.

It is important to take into consideration that, thanks to the simplifications, while a technician with a minimum of experience can complete a simple simulation with the CE3X program in one working day, the simulation with IDA-ICE requires a greater knowledge of the computer program, and two or three days are needed if the simulation is to be completed in maximum detail.

However, this is inevitably reflected in the result. The results obtained in the two simulations carried out in the thesis show differences in the calculated energy usage of around 14%. In addition, the thesis includes an assessment of the proposed improvement measures, which also make it possible to study the different behaviour of the two computer programs when faced with the same reference situation. Improvements in lighting efficiency is evaluated as well as improvements in the thermal envelope or building enclosure. While the result obtained ensures that improvements in lighting are always cost-effective, any improvements in the thermal envelope (changing windows, for example), from an economical point of view, are not profitable.

The result obtained is consistent with the information found in the literature review (this type of simulation must be always considered within a wide margin of error). The criterion used by each technician in how the input data are used or introduced in the software can make the results vary an additional high percentage. Moreover, sometimes it is not possible to determine which criterion used is more valid.

For all these reasons, the comparison made in the thesis allows to state that the CE3X programme allows compliance with the Spanish regulations and gives fast results that can be considered relatively reliable in some cases, after comparing them with the results obtained from IDA-ICE. However, IDA-ICE allows results of higher quality.