This paper develops a methodology for calculating the technical and economic indices of a VVER-1000 NPP unit within the upper level control system (ULCS) of an APCS. It substantiates its use as a tool for operational technical diagnostics of process equipment conditions. It is shown that decomposing the unit's thermal efficiency deviation from the standard value into components attributed to individual elements of the thermal circuit, computed in each cycle, yields diagnostic information that localizes the source of efficiency decline. A multi-level computational architecture is described, comprising arrays of discrete and analog signals, standard reference constants, intermediate and final arrays, and a hierarchy of temporal aggregation over operational, shift, daily, and monthly intervals. Relationships linking input-measured values to output integral indicators and to the diagnostic components of heat excess are formalized. Considerable attention is devoted to ensuring the reliability of results under incomplete and unreliable input signals. A quantitative measure of criticality is proposed, enabling the ranking of measurement channels by their impact on calculation completeness and justifying redundancy and managed-degradation strategies. A distinctive feature is the recalculation of normative characteristics in each cycle based on the current mode, ensuring accurate diagnostics across flexible operating regimes. Results of testing on archival data from operating units and of industrial implementation are presented. The proposed approach integrates the functions of technical-economic analysis and technical diagnostics within a single software package, without requiring additional measurement channels.