Calculating generation adequacy indices (GAI) is performed not only for planning problems but also for on-line computation. Practically, the main mathematical method for calculating GAI of electrical grid with complex structure is Monte-Carlo simulation (MCS). It provides sufficiently accurate results, but the computation is time-consuming for large systems. On-line GAI computation requires an accurate method with low computation time. For that reason, analytical methods for GAI calculations have been proposed. Most of the proposed analytical methods include a lot of constraints and suppositions in calculating GAI. comparative analysis of the case study results helps to estimate efficiency for the analytical methods. Test results have been calculated by MCS and by analytical calculations based on the supplied demand algorithm. Analytical solution was implemented by the C# and VBA programming language. Input data includes generation set data, generation and load probability indices. The comparison of supplied demand algorithm and MCS in calculating GAI has been made. Guidelines for combined using of analyzed methods have been formulated. Investigation results allow to select proper method for calculating GAI.