Adolescents are an important population to target with energy-saving interventions: Not only are adolescents high consumers of energy, but they are also the adult consumers of the future. In the present study, an online energy-saving intervention was developed based on implementation intentions, a widely used psychological behavior change technique. A total of 180 adolescents, aged 13 to 15 years, were recruited using purposeful sampling and were allocated to either the intervention or control condition. A significant increase in adolescent’s self-reported energy-saving behavior was found among adolescents who had received the intervention, which was sustained at the 6-week follow-up. However, some adolescents were more affected by the intervention than others; adolescents who already actively engaged in energy saving (as identified by their readiness to change prior to the intervention) reported a significant increase in energy-saving behaviors as a consequence of participation in the intervention, whereas those who were not already saving energy did not.