How it is calculated: For a fixed array optimized for year-round production, a practical rule of thumb is to use a tilt close to the location’s absolute latitude. For this simple calculator, summer tilt is estimated as latitude minus 15°, while winter tilt is estimated as latitude plus 15°. Results are limited to the physical range of 0° to 90°. Panels generally face toward the equator: south in the Northern Hemisphere and north in the Southern Hemisphere. Exact optimum tilt can differ because of local solar resource, shading, roof orientation, climate and electricity-use patterns.