7 Solar Resource and Irradiance Assessment

Solar Resource and Irradiance Assessment is the process of quantifying how much solar energy is available at a specific location and how that energy varies over time, providing the fundamental input used to predict the energy output of a photovoltaic or solar thermal system. It draws on measured and modeled irradiance data, atmospheric and geometric factors affecting sunlight intensity, and site-specific conditions such as shading and orientation to establish a reliable estimate of the solar resource a system will actually be able to capture.

Types of Solar Irradiance

Direct, Diffuse, and Global Irradiance

Solar radiation reaching the ground is categorized into direct normal irradiance, the component arriving in a straight line from the sun, diffuse horizontal irradiance, the component scattered by the atmosphere and clouds before reaching the surface, and global horizontal irradiance, the total of both components measured on a horizontal surface. The proportion of direct to diffuse irradiance at a site depends heavily on local climate and cloud cover, and it influences which system types, such as tracking arrays that benefit most from strong direct sunlight, are best suited to a given location.

Plane-of-Array Irradiance

Because most arrays are mounted at a tilt rather than lying flat, assessment must convert horizontal irradiance measurements into plane-of-array irradiance, the amount of solar energy actually striking the tilted collector surface, accounting for the geometric relationship between the sun's position and the panel's orientation at each moment of the day.

where the beam irradiance component is scaled by the cosine of the angle of incidence between the sun's rays and the panel's surface normal, and added to the diffuse irradiance component reaching the tilted surface.

Quantifying the Resource

Peak Sun Hours

A widely used simplification for solar resource assessment is peak sun hours, defined as the number of hours per day during which sunlight would need to arrive at a standard reference intensity to deliver the same total daily energy as the actual, variable irradiance observed at the site. This metric allows straightforward estimation of expected system output without requiring detailed hour-by-hour modeling for every calculation.

Annual and Seasonal Variation

Solar resource assessment accounts for the substantial variation in available sunlight across the year, driven by changes in day length and the sun's angle in the sky between summer and winter, information that is essential for accurately predicting a system's monthly and annual energy production rather than relying on a single average figure.

Site-Specific Assessment Factors

Shading Analysis

Beyond regional climate data, an accurate resource assessment requires site-specific shading analysis, identifying obstructions such as trees, adjacent buildings, or roof features that block direct sunlight during particular hours or seasons, since shading losses can substantially reduce the usable solar resource even at a site with an otherwise excellent regional irradiance rating.

Orientation and Tilt Optimization

Resource assessment includes evaluating how a collector's azimuth and tilt angle affect the captured irradiance relative to the theoretical optimum for the site's latitude, since deviations from the ideal orientation, often necessitated by roof geometry, reduce the effective resource available to the system compared to an ideally oriented reference array.

Data Sources and Modeling

Ground-Based Measurement and Satellite Data

Solar resource data is derived from ground-based measurement stations using pyranometers and other instruments, as well as from satellite-derived irradiance models that estimate surface irradiance from cloud cover observations, with satellite-derived data providing broader geographic coverage while ground measurements offer higher accuracy at specific locations.

Typical Meteorological Year Data

For system performance modeling, engineers commonly use a typical meteorological year data set, a constructed year-long record assembled from historical data to represent typical, rather than extreme, solar and weather conditions at a location, providing a standardized basis for comparing expected performance across different system designs or locations.