- We derive, evaluate and validate comprehensive analytic modeling of the energy flows in parabolic trough solar thermal power plants. The analytic formulae are straightforward to implement and evaluate, relating to the heat transfer within and from the solar concentrators (including transients, mainly overnight heat losses), and the impact of solar field operation on turbine power and efficiency. Prior numerical simulations used to design solar thermal power systems have either been proprietary or devoid of a fully-reported source code - hence inaccessible or problematic for widespread use. Also, the dependence of these simulations on extensive numerical procedures does not provide a transparent physical picture that grants a clear understanding of how component and system performance vary with the principal operating and input variables - a drawback overcome by the analytic approach presented here. Published experimental measurements of sufficient extent to permit meaningful comparisons between theory and experiment for such solar thermal power plants are exceptionally limited. This narrow data base is used for model validation on both a monthly and an hourly basis. The analytic model is then applied to evaluating a solar power plant now being planned for northeast Brazil, also highlighting the energy-delivery advantages of low-latitude locations.