Almost all energy systems feature a multitude of length and time scales. E.g., in combustion the physical and chemical processes relevant to the chemical kinetics occur on the molecular scale, while macroscopic transport processes may involve length-scales of many meters. Similarly, in the electricity grid, individual user components may only be a couple of cm in size while electricity grids span whole continents. Dealing with systems involving multiple scales is notoriously difficult. We use a range of numerical and analytical techniques to address these issues.