The concepts of power-source and power-load elements are considered. The consideration of losses shows the two-valued characteristics of such elements. The interpretation of typical quadratic expressions, as transformations of projective geometry, is used. The projective transformations preserve an invariant in the restricted one-valued area of the characteristic; there is a cross ratio of regime parameters. Direct recalculation formulas with the group properties are obtained. The regime parameters are represented relatively to the characteristic values. The parameter changes are introduced in another way unlike changes in the form of increments. The obtained results are useful for the power supply system monitoring.