The conversion of heat into electricity through the thermoelectric effect and light into\ud electricity through photovoltaic solar cells both allow useful amounts of power for a\ud range of ICT systems from a few milli-Watts (mW) for autonomous sensors up to kiloWatts\ud (kW) for complete ICT computing or entertainment systems. Photovoltaics at the\ud large scale can also be used to produce MW power stations suitable for the sustainable\ud powering of high-performance computing (HPC) and dataservers for cloud computing.\ud This chapter provides a background to the physics of operation of both types of sustainable\ud energy sources along with the fundamental limits of both technologies. The present\ud performance is presented along with promising research directions to allow for a comparison\ud of the useful power along with the limits for deployment of each approach to\ud power ICT devices and systems. Finally, the developing field of thermal photovoltaics is\ud reviewed, where the overall thermodynamic conversion efficiency of turning light into\ud electricity and useful heat can be increased through the addition of thermoelectrics or\ud heat transfer modules to a photovoltaic cell.