The conversion of structural dynamic strain into electric power using piezoelectric transducers to power microelectronic devices and wireless sensor nodes for structure health monitoring has been receiving growing attention from academic researchers and industry. Harvesting electric energy from vibration and storing it in an external infinite life-span capacitor is a proposed technique to eliminate the drawbacks of using conventional finite life-span batteries. Optimisation of the harvested power is an important research topic to ensure an endless power source with sufficient flow of electricity. This paper concerns optimisation of energy harvesting for composite shells stiffened by beams, with discrete flexible composite piezoelectric sensors bonded to the surface and located optimally. A homogenous composite shell stiffened by beams with a bonded piezoelectric transducer connected to an external resistive load is modelled using three-dimensional solid finite elements. An efficient and effective placement methodology is proposed to find the optimal locations of piezoelectric sensors based on the maximisation of average percentage sensor effectiveness as an objective function. This study is firstly verified against published work for a cantilever flat plate and beam, and then implemented to optimise the energy harvesting for a composite aircraft wing at structural frequencies during flight. The results show a high reduction in computational effort and improved effectiveness of the methodology to optimise energy harvesting for complex and large-scale structures compared with alternative methods. Furthermore, the harvesting power obtained from optimal sensor distribution shows promise to be sufficient to activate wireless sensor nodes for health monitoring.