For tracking and stationary flat plate (nonconcentrating) solar collectors, we develop easy-to-use, closed form, analytic formulae for yearly collectible energy as a function of radiation threshold. Primary applications include central-station photovoltaic systems. These correlations include the explicit dependence on: yearly average clearness index, latitude, and ground cover ratio (shading effects), and are in excellent agreement with data-based results for 26 U.S. SOLMET stations. They also incorporate appropriate functional forms that ensure accurate results for photovoltaic system design and, in particular, for systems with buy-back thresholds. Both beam and diffuse shading are treated properly and diffuse shading is found to represent a 2%-6% loss that has systematically been ignored in past studies. Sample sensitivity studies illustrate evaluation of the energetic advantage of tracking vs. stationary deployment and its significant dependence on ground cover ratio. The impact of isotropic versus anisotropic modeling of diffuse radiation is quantified and shown to give rise to non-negligible differences (up to 10%) in yearly collectible energy. We also determine an optimal tracking strategy for two-axis trackers which, however, is found to yield a 0%-2% energetic advantage relative to conventional normal-incidence tracking.