Solmetric offers its free online Annual Insolation chart, which illustrates the effects of an array’s tilt angle and azimuth (deviation from true south) on the amount of annual solar radiation that strikes it. Data is available for more than 1,000 sites across the United States. This program is directly applicable to grid-tied PV systems, in which maximizing the amount of energy produced by a system is desirable. Since net-metered systems do not need to store energy and often provide the greatest benefit by maximizing their annual production, PV installers can refer to these charts to determine how a particular roof pitch or building orientation might affect array production.
While this tool can provide some general comparisons between different orientations of solar heating systems, caution must be exercised. Unlike net-metered PV systems, the energy value produced by a SWH system often depends upon storage and always depends upon the quantity of hot water used in the building. Trying to maximize annual production in an SWH system can lead to system overheating in the summer and minimal production in the winter because the amount of energy that strikes a surface is maximized by favoring the time of year when the most solar radiation is available. Instead, SWH system design seeks to match system production with hot water consumption. A standard residential SWH system is quite similar to an off-grid PV system—the match between production and storage is crucial.
An important lesson that can be learned from the chart is that minor deviations from a true south azimuth and tilt set at the location’s latitude have minimal effects on the amount of solar energy that strikes the array. Constructing elaborate designs to orient a collector array to true south or at an “optimal” tilt angle is often not worth the expense or aesthetic sacrifice.