Measuring Solar Radiation on a Tilted Surface

When specifying collector tilts and orientations, the notation used in this book will be GTI (Tilt, Orientation), where Tilt is given in degrees from the horizontal, and Orientation is in degrees measured eastward from true north (0°). For example, total irradiance on a surface tilted up 30° from a horizontal surface and facing true south (180°) is designated GTI(30°,180°).