The calculation of the operation metrics of an accelerator serves several purposes: it quantifies the improvement of a specific facility over time, or it is used to compare the performance of similar facilities. In the first case one should select an operation metrics that is close to the requirements of the users of the facility \cite{Luedeke:2014aa}. For the latter case the operation metrics of the compared facilities should follow a common standard. In particular the beam availability is often used to compare light sources. While these statistics are published for most light sources, very few facilities do add exact definitions on how these numbers are calculated. A survey of several light sources revealed \cite{L_deke_2009} that the calculation of these metrics do vary considerably. The conditions under which beam is considered available are often defined in common sense terms and even if there are formal definitions, these do differ between the facilities. But if beam availability is not identically defined for the facilities, then these metrics are not useful for a meaningful comparison. Our aim is to propose a simple, well-defined, formal operation metrics for storage ring light sources, to make the reliability of these facilities comparable. The metrics itself clarifies for each facility the beam parameter promised to the users, and the statistical data do reveal, how well these promises are kept.