Solar irradiance

Solar irradiance (SI) is a measure of the intensity of electromagnetic radiation reaching a unit of area, expressed as the power per unit area, W/m2. When solar irradiance is integrated over time it is solar irradiation (also insolation), and is measured as the radiant energy per unit area, J/m2. The irradiance at the surface of the Earth will vary by many factors, primarily latitude, topography, weather, cloud cover and pollution.

TSI = Total Solar Irradiance (received at upper atmosphere) = 1.361 kW/m2 (mean TSI of an area perpendicular to incoming radiation, at one AU = Solar Constant)

DNI = Direct Nominal Irradiance (received at surface) = ~<1.0 kW /m2

(DNI = TSI – diffuse solar radiation (scattering or reflection of radiation by the atmosphere))

DHI = Diffuse Horizontal Irradiation (scattered by atmosphere)

GHI = Global Horizontal Irradiation (sum of indrect irradiation and diffuse horizontal irradiance)

= DHI + DNI*cos(z)

z = zenith angle

Examples of GHI
Lugano, southern Switzerland: GHI = 1200 kWhm-2.yr

Riad, Saudi Arabia: GHI = 2500 kWhm-2.yr

London: 1.08 – 4.91 kWh/day m-2 for 38°-angled panel (optimally adjusted)

Rome: 3.46 – 7.68 kWh/day m-2 for 48°-angled panel (optimally adjusted)

Marrakesh: 5.14 – 7.26 kWh/day m-2 for 58°-angled panel (optimally adjusted)

Melbourne, Australia: 3.09 – 6.23 kWh/day m-2 for 52°-angled panel (optimally adjusted)