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For the latter case one needs identical operation metrics for the compared facilities.   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 publish exact definitions on how these numbers are calculated.   A survey of several light sources revealed~\cite{Luedeke:2009aa}, 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 all compared 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.