This work estimated the energy required to dewater a clay suspension, considering the total energy input received by the suspension from the dewatering device, the bond strength between adjacent water and solid surface, and the intracake friction loss. The centrifugal dewatering and consolidation dewatering were the testing means, and a UK ball clay suspension coagulated with alum was the testing sample. At the initial stage of centrifugal dewatering, most energy input was used to overcome process irreversibilities other than intra-cake friction, giving low-energy efficiency. To rotate faster or to flocculate at optimal dose needed more energy to dewater. On consolidation, most input energy was consumed to break down the bond strength to a critical residual water content, beyond which the friction loss became dominant. The methods presented herein provided a quantitative index to evaluate the efficiency of real dewatering process from an "ideal" dewatering system.