Anaerobic digestion promotes simultaneous sludge stabilisation and bioenergy generation. Increasing the net energy production would contribute in reducing the energy demand of sewage treatment plants. Thermophilic digestion may be used to upgrade (conventional) mesophilic digestion, a major drawback being increased energy requirements. The objective of this study is to compare the mesophilic and thermophilic sludge digestion from an energy perspective. Data from laboratory-, pilot- and full-scale digesters are used to compare the energy balance and ratio of full-scale systems. The results highlight the importance of sludge characteristics on the effectiveness of the process, and the need to recover energy from digestates in thermophilic digesters. Net energy production is comparable in thermophilic with half the retention time of mesophilic systems (10-15 vs. 20-30 days).