Applying nanotechnology to efficiently harvest energy from ambient environment is of great importance owing to its broad application prospects. However, the mechanical analysis of harvesters, especially at the nano-scale, is challenging. This study establishes a general thermal-mechanical-electrical coupling model of a beam-type harvester to exactly depict the working performance of a harvester at the nanoscale. The model considers both the surface effect and flexoelectricity. The size-dependent thermalmechanical-electrical coupling model is mathematically depicted by introducing an additional thin layer, whose material property is represented by a surface parameter. This model can be reduced to the classical cases if some specific assumptions are made. After the validation, a systematic numerical simulation is carried out for a PZT-5H/silicon composite harvester, which focuses on the performance improvement. The size-dependent property is evident when the surface layer-to-bulk thickness ratio is greater than approximately 10(-2). Correspondingly, a critical beam thickness that quantitatively distinguishes the surface effect from the macro-mechanical behaviors can be proposed if the surface parameter is determined. The proposed general model can be a useful tool to explain the inherent physical mechanism, structural design and eventually system optimization for harvesters both in the nano- and macro-scale. (C) 2019 Elsevier Ltd. All rights reserved.