Endesa procures the amount of energy equivalent to that consumed by the customer's supply from power plants that generate using renewable sources and high efficiency co-generation.
Evidently, it cannot be guaranteed that the energy going into the customer's supply comes from a renewable source, since within the grid energy from all sources is mixed together. What can be guaranteed is that for the quantity of energy consumed by the customer, an equal amount has been generated using renewable sources and high efficiency co-generation.
Renewable Energies: Hydro, wind, solar...
High efficiency co-generation: When an industry needs to consume resources to generate heat (for example gas), it uses these same resources to generate electricity.
A small number of additional resources are used in the production of this electricity, which would have been required regardless for the production of heat alone.