Optical burst switching (OBS) with the nature of optical bypass is one of the potential techniques for future green optical internet. Due to the fact that main operations perform at edge nodes, the energy efficiency of OBS is directly affected by parameters setting, but it has been rarely reported to date. In the article, based on low power idle (LPI), the transition of burst is comprehensively analyzed and the quantitative energy model is established. Further, the energy efficiencies of extra offset time (EOT) and fixed assembly size (FAS) are compared under the target loss probability. The numerical results show that, with LPI model, EOT is superior to FAS in energy consumption because a 48.86% saving is gained.