The paper considers a problem of calculating power system adequacy indices (AI) for an united power system in general and a power shortage distribution (PSD) problem in particular. It shows that principal difference in resulting AI calculated by diverse software packages (SP) is due to different PSD strategies employed by these SP. One of the main factors of the AI difference is concerned with solution uncertainty driven by applied mathematical model and approach for the PSD problem. Especially it concerns linear programming method and similar approaches. Another AI difference factor is caused by different load shedding strategies and their mathematical formulation. This factor involves “proportional”, “local” and “nonlinea” load shedding strategies. The paper describes mathematical models for the most often used PSD strategies. It recommends nonlinear PSD strategy and proportional PSD strategy with active frequency effect for practical calculations.