(Abbreviated as wt. %. ) Weight percent is widely used in hydrogen storage research to denote the amount of hydrogen stored on a weight basis (the term mass % is also occasionally used). The term can be used for materials that store hydrogen or for the entire storage system (e.g., material or compressed/liquid hydrogen as well as the tank and other equipment required to contain the hydrogen such as insulation, valves, regulators, etc. ). For example, 6 wt. % on a system-basis means that 6% of the entire system by weight is hydrogen. On a material basis, the wt. % is the mass of hydrogen divided by the mass of material plus hydrogen.