Energy efficiency for video communications and video-on-demand streaming is essential for mobile devices with a limited battery capacity. Therefore, hardware (HW) decoder implementations are commonly used to significantly reduce the energetic load of video playback. The energy consumption of such a HW implementation largely depends on a previously finalized standardization of a video codec that specifies which coding tools and methods are included in the new video codec. However, during the standardization, the true complexity of a HW implementation is unknown, and the adoption of coding tools relies solely on the expertise of experts in the industry. By using software (SW) decoder profiling, we are able to estimate the SW decoding energy demand with an average error of 1.25%. We propose a method that accurately models the energy demand of existing HW decoders with an average error of 1.79% by exploiting information from software (SW) decoder profiling. Motivated by the low estimation error, we propose a HW decoding energy metric that can predict and estimate the complexity of an unknown HW implementation using information from existing HW decoder implementations and available SW implementations of the future video decoder. By using multiple video codecs for model training, we can predict the complexity of a HW decoder with an error of less than 8% and a minimum error of 4.54% without using the corresponding HW decoder for training.