In this work, the design, fabrication and characterisation of a millimetre-size photovoltaic (PV) energy source is described. The fabrication process is based on p-type silicon-on-insulator wafers. It is shown that scaling up the number of cells from 9 to 169 makes the open-circuit voltage to increase from 3.6 to 101.5V with generated power densities ranging from 2.07 to 6.7mW/cm2 under 100mW/cm2 standard AM 1.5 Global Spectrum. A prototype consisting of a PV mini-module connected to a mini-battery has been assembled, modelled and haracterised. The results show that the mini-PV source can be used in low power applications.