AVAILABILITY is defined as the probability that the system is operable (in “UP” state) at any time.
Availability for Series. (Series, if the system goes down if one unit goes down)
Atotal = A1 * A2*A3* … *An
where: An = (1 - Un)
Un = (number of hours that unit n is down) / (total number of hours)
Availability for parallel. (Parallel if the system will fail ONLY if and only if all of the units fail)
Atotal = 1-(U1*U2*U2*…*Un)
where: Un =(number of hours that unit n is down) / (total number of hours)