Direct Solar Radiation ($Q_i = Q_o \times D \times A \times \cos(\alpha)$):

Where:

• $Q_i$ is the direct solar radiation, in kilowatts (kW).
• $Q_o$ is the solar constant, in kilowatts per square meter (kW/m²).
• $A$ is the absorber surface area, in square meters (m²).
• $D$ is the transmission factor (0.06 – 0.82).
• $\alpha$ is the angle between a line perpendicular to the surface and the direction of radiation, in degrees.