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Future power networks will include various local sources (LS)s as alternatives to centralized power generation in sustaining local loads. The cost of electrical power at LSs depends on the specific kind of the source, e.g., operating on renewable energy, using energy storage equipments, being a fuel cell, etc. In general, the cost will be proportional to the provided active power. In this paper we aim at optimizing the usage of LSs by carefully selecting current injection in order to minimize a total cost which includes the cost of the main source. The optimization is subject to constraints related both to LSs capabilities (i.e., maximum provided active power) and to the quality of the power provided by the main source (i.e., minimum power factor met at the electric utility interface). The system is described as a linear network characterized also by line impedances. We find that the problem is quadratic with quadratic constraints and we resort to numerical methods for its solution. An iterative, suboptimal solution is also investigated, suited to be implemented in a distributed fashion.