The aim of this thesis is the study of the total and spectral solar irradiance variability induced by the presence of small magnetic elements that emerge into the solar photosphere. It is important to study changes in the solar energy output because they reflect the existence of several physical processes in the solar interior, their interpretation helps to understand the solar cycle and because of their influence on the terrestrial climate. The work presented in this thesis is exclusively based on data provided by the SOHO spacecraft, specifically by the VIRGO and MDI instruments.Irradiance variations produced on the solar rotation time-scale are due to the passage of active regions across the solar disk. However, the origin of variations on the solar cycle time-scale is under debate. One of the most controversial aspects is the long-term contribution of the small magnetic elements conforming faculae and the network. Their identification and contrast measurement is difficult and, consequently, their contrast center-to-limb variation (CLV) remains poorly defined in spite of the fact that its knowledge is essential to determine their contribution to variability.In this work we have studied the contribution of small photospheric magnetic elements (those with a positive contribution to variability), both on short, i.e. solar rotation, and long, i.e. solar cycle, time-scales. By analyzing the evolution of an isolated active region (NOAA AR 7978) during several Carrington rotations, we have evaluated the variations in luminosity induced by this facular region during the 1996 minimum of activity. Simultaneous photometric and magnetic data from the MDI instrument have been combined in order to study the contrast of small scale magnetic features and its dependence both on position and magnetic field, as well as its evolution along the rising phase of solar cycle 23.The study of the solar variability has required reduction and analysis of the employed MDI and VIRGO data. These data had to be converted from level 0 (raw data) to level 2 (scientifically useful data), since solar variations were hidden by instrumental effects. We developed original algorithms to correct instrument-related effects from the data, such as filter degradation and the variation of the limb darkening with distance. The determination of the contrast of magnetic features also required the development of an algorithm in order to identify the surface distribution of those small features present over the solar disk.By analyzing irradiance variations induced by the small magnetic features that emerge into the solar photosphere we have concluded that:· active region faculae and the magnetic network present very different contrast CLV's, therefore, their contributions to irradiance variability are distinct; as a consequence, both contributions need to be taken into account separately when reconstructing variations of the solar irradiance.· the functional dependence on position and magnetic signal of the facular contrast is time independent; this suggests that the physical properties of the underlying flux tubes do not vary with time.· network elements are bright over the whole solar disk and have proved to be the dominant population along the solar cycle; this implies that their contribution to long-term irradiance variations is significant and needs to be taken into account.