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