The conversion of abundant but intermittent forms of energy, such as solar or wind, into more technologically accessible electrical or chemical energy, is typically driven by electron transfer processes at embedded material interfaces. To build optimal composite materials for energy conversion requires detailed understanding and exquisite control of the nanoscale details of these interfaces. The necessary guidance to drive such synthesis and fabrication efforts defines a challenge for direct characterization of these processes, as they happen, with techniques that are often best suited to open interfaces in the realm of surface science. For example, X-ray spectroscopy, which provides insight on local, element-specific chemistry and coordination, can be extremely surface sensitive, but has only recently advanced to reveal interfacial chemistry at the solid-liquid interface. Moreover, the interpretation of such measurements relies both on accurate modeling of condensed phase interfaces and of the associated observables. At the Molecular Foundry, we combine dynamical modeling of interfacial function with direct simulation of X-ray absorption and photoemission spectra. Our simulations provide nanoscale structural details of interfaces and can reveal unique spectral signatures of the interfacial phase or specific interfacial phenomena that may lie hidden in characterization measurements. The combination of such theoretical modeling with in situ characterization is vital to advance research on energy conversion and storage. Here we will discuss a range of recent applications in the areas of electrochemical energy storage, photoelectrochemistry, and gas storage via chemical conversion. Typically, we find that materials interfaces do present unique spectral signatures that are measurable indicators of nanoscale interfacial function, which can reveal intrinsic bottlenecks and paths to advancement.