Continuous integration is essential for software quality, yet the energy footprint associated with its frequent execution has largely remained invisible. We provide the first comprehensive baseline of CI energy use through a largescale study of 204 open-source Java projects with repeated measurements under Maven and Gradle. Our results show that energy use is highly skewed: while most projects consume energy modestly, a small number of “CI-intensive” systems can reach annual CI energy footprints of hundreds of kilowatt-hours, which is comparable to a quarter of an average EU household's electricity use. We further show that immediate, practical savings are possible: simply enabling dependency caching cuts energy by 30 % on average in some Maven projects and by over 90% in some Gradle cases. These findings matter not only for individual developers, but also for large organizations that run thousands of builds. In those settings, even small inefficiencies can add up to very large energy costs. By exposing where energy is consumed and how to reduce it, our study establishes an actionable foundation for greener CI pipelines.