The High Efficiency Video Coding (HEVC) standard provides up to 40% bitrate savings compared to the state-of-art H.264/AVC standard for the same perceptual video quality. Power consumption constraints represent a serious challenge for embedded applications based on a software design. A large number of systems are likely to integrate the HEVC codec in the long run and will need to be energy aware. In this context, we carry out a complexity study of the HEVC coding trees encoding process. This study shows that the complexity of encoding a Coding Unit (CU) of a given size has a non trivial probability density shape and thus can hardly be predicted with accuracy. However, we propose a model that linearly links the ratios between the complexities of coarse-grain and lower-grain CU encodings with a precision error under 6%. This model is valid for a wide range of video contents coded in Intra configurations at different bitrates. This information is useful to control encoder energy during the encoding process on battery limited devices.