We have illustrated the important role played by the nanoscale materials in three-up-to-date energy topics.1/The solar-to-electrical energy conversion in photoelectrochemical cells: we have shown two favorable situations for which photoelectrochemical cells using porous nanocrystalline films have high efficiencies.2/The electrical energy storage in rechargeable rocking-chair lithium batteries: these systems, which use nanocrystalline materials, might be the next generation of rechargeable batteries showing higher capacity, cyclability, and safety than conventional lithium ion batteries.3/The energy saving with efficient electrochromic windows using nanocrystalline materials.