The Integrated Starter Alternator (ISA) needs to cater for a very wide range of torque and speed, in both motor and generator modes and in different functionalities, such as engine starting, alternator, regenerative braking and torque boost. Furthermore, each of these functionalities, being dependent on the drive cycle, means that determining the optimal ISA torque-speed characteristics is a challenging problem. In this paper, a Monte-Carlo simulation has been reported to find the most frequently operated regions in the torque-speed plane of an ISA, using stochastic drive cycles. This has been used to determine the optimal torque-speed characteristic to inform the ISA design specification. In turn, this gives a more robust coverage of different driving patterns compared to a conventional driving cycle-based design.