Storage ring light sources aim for high operational reliability. Very often beam availability is used as an operation metric to measure the reliability. A survey of several light sources reveals that the calculation of availability varies significantly between facilities. This complicates useful comparisons of reliability. Furthermore the beam availability typically does not provide insight regarding reliability of beam characteristics such as orbit- and beam size stability.

The authors propose specific metrics to evaluate the reliability of storage ring light sources; these metrics allow detailed and meaningful comparison across facilities. Such comparisons are useful to further optimize the reliability of storage ring light source facilities.