The problem of determining the optimal size of distributed generation units in the microgrid can be viewed as a multi-objective optimization problem, currently being solved by using different optimization techniques. Determining the optimal size of distributed generation units in the microgrid is very important in order to justify the economic cost of the investment, to minimize the operational costs and to reduce their negative impact on the distribution network. The doctoral thesis presents a multi-objective optimization algorithm for conducting comprehensive techno-economic analyses with the goal of determining the optimal size of distributed generation units, especially PV plants and battery energy storage systems, in microgrids. Because the objective and constraint functions are linear functions, MILP is used for solving the described optimization problems, and the optimization itself is implemented in the optimization packet tool FICO Xpress. The main precondition for providing ancillary services in the microgrid, opening new business opportunities for microgrid owners, is determining the scope of ancillary services which can be offered by the microgrid as well as planning the operation of the microgrid itself under such conditions. Thus, one of the research objectives was to define an optimization model for planning the scope of ancillary services in microgrids with flexible loads and battery energy storage systems. The results of the conducted analyses show that investments in microgrids with flexible and non-flexible loads, PV plants and battery energy storage systems, is still not economically viable without incentives, but that the business opportunities provided by ancillary services could make it feasible through the additional revenue they provide.