The Average Service Unavailability Index (ASUI)[1] is a reliability index commonly used by electric power utilities. It is calculated as [math]\displaystyle{ \mbox{ASUI} = \frac{\sum{U_i N_i}}{\sum{N_i} \times 8760} = 1- \mbox{ASAI} }[/math]
where [math]\displaystyle{ N_i }[/math] is the number of customers and [math]\displaystyle{ U_i }[/math] is the annual outage time (in hours) for location [math]\displaystyle{ i }[/math]. ASUI can be represented in relation to SAIDI (where annual SAIDI is given in hours) as
[math]\displaystyle{ \mbox{ASUI} = \frac{\mbox{SAIDI}}{8760} }[/math]