This paper presents analytical expressions for the annual energy losses on dc and ac cables, the annual energy losses on the step-up transformer, and the optimum transformer size for fixed-mounted photovoltaic (PV) plants of any size. These expressions are based on some unknown parameters, which can be easily estimated from data provided by the equipment (inverters or transformer) manufacturer and from available data in any meteorological database. The analytical expressions are meant to be used by the design engineers for comparing-from the energetic point of view-different cable cross sections and transformer sizes with various power loss classes or cooling medium, without having to perform multiple detailed simulations, as the current practice is. The validity of the derived expressions was tested against measurements taken in two existing 1003.52- and 491.4 kW p PV installations during the past 2 years. The proper use of the derived expressions during the design phase of a PV system is demonstrated through a detailed working example.