Euphorbia characias has drawn much attention as a potential bioenergy crop given its considerable amount of latex, rich in hydrocarbon-like compounds, and its ability to grow in large areas of semiarid lands. Compositions of major constituents with an energy value have been determined for the three phenological stages of this plant (preflowering, flowering, and postflowering) and different irrigation treatments. Metabolites from both nonpolar and polar extracts have been identified and quantified by GC-MS, GC-FID, HPLC-ELSD, and UPLC-PDA-MS. The results highlight that the end of the flowering period is the optimal harvesting time to maximize the yields of E. characias as a potential energy crop. The total water requirements to obtain the maximum yields of hexane- and methanol-extractables were determined for its annual development cycle.