Abstract. Estimation of potential annual direct incident radiation has traditionally required numerical integration with simulation models. As an alternative, we present convenient equations for use in spreadsheet, GIS, and database applications. Input variables are latitude, slope, and aspect. The equations apply to 0–60° north latitude, slopes from 0–90°, and all aspects. By transforming aspect, the equations can also be applied as an index of heat load, symmetrical about a northeast to southwest axis.