Storage ring light sources aim for a high operation reliability. Very often beam availability is used as an operation metrics to measure the reliability of the accelerator. A survey of several light sources revealed that the calculation of availability varies significantly between facilities. This prevents a useful comparison of reliability. Furthermore the beam availability does not provide inside about the reliability of beam characteristics like orbit- or beam size stability.

The authors propose a specific metrics to evaluate the reliability of storage ring light sources; a metrics that will allow a detailed and meaningful comparison across facilities. The authors are convinced that such a comparison will be useful to further optimize the reliability of storage ring light sources.