1. Code embodied in a computer readable storage medium, when executed by a computer operable to: perform for each operational unit of a plurality of operational units of a system: receive, from a user interface, one or more operational reliability metrics for each operational unit; designate that one or more spare reliability metrics for at least one spare unit associated with the each operational unit are similar to the one or more operational reliability metrics, the spare unit comprising a cold spare unit or a hot spare unit; calculate an availability value for the system in accordance with the one or more operational reliability metrics and the one or more spare reliability metrics; and output the availability value to the user interface.