The optimal power shortage (PS) allocation among parts of an interconnected power system (IPS) is a key procedure in adequacy indices calculation. The optimization criteria, which include objective functions, constraints and input arguments, may vary depending on a problem statement. It was shown that the most appropriate criteria are the ones of minimal operational costs and proportional allocation of a PS among power systems, and the most significant constraints are ones of interconnection lines capacities. The specificity of input arguments selection was considered in relation to the nonlinear programming methods used in optimization procedures. The optimization problem was demonstrated to be reduced to a quadratic programming problem in the majority of objective functions and input arguments combinations.