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\section{INTRODUCTION}   The calculation ofthe  operation metrics of for  an accelerator serves several purposes: it quantifies the can quantify  improvement of a specific facility over time, or it is can be  used to compare the performance of similar facilities. In the first case one should selectan  operation metrics that is close to in line with  the user  requirements of the users of for  the facility~\cite{Luedeke:2014aa}. For the latter case theoperation  metrics of for  the compared facilities should follow a common standard. In particularthe  beam availability is often used to compare light sources. While these statistics are published for most light sources,very  few facilities do add exact supply  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 this metric varies  considerably. The conditions under which beam is considered available are often defined in common sense terms terms,  and even if there are formal definitions, these do they  differ betweenthe  facilities. But if beam availability is not identically defined for the facilities, defined,  then these metrics are the metric is  not useful for a meaningful comparison. Our aim is to proposea  simple, well-defined, formal operation metrics for storage ring light sources, to make the reliability of these facilities comparable. The metrics itself clarifies serve to clarify  for each facility the beam parameter promised to the users, and the statistical datado  reveal, how well these promises the standards  are kept. met.