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Although some works have been done for energy efficiency in cooperative communications, most of them don't consider reliable cooperative communications. However, reliable cooperative communications require re-transmissions, which will increase energy consumption. In this paper, instead of being interested in outage behavior and error analysis, we investigate the energy efficiency of reliable decode-and-forward cooperative communications in terms of a metric of bits per unit energy. Our analytical results reveal that the energy efficiency in reliable cooperative communications is concave with regard to the power level and the power ratio of the source to the relay, which determine the energy efficiency jointly. The processing energy at the receiver does not affect the concave relationship. These results may lead to the design of optimal power assignment and relay selection schemes.