The power of the sun striking the Earth at its peak, at midday on a cloudless day, is 1000W per m². However, taking into account tilt at latitudes between 50°N and 60°N seasonal factors and average cloud cover, a south-facing roof in the UK can generate power from sunshine of 110W / m² whilst the average raw power generated by sunlight at ground level is likely to be lower still at 100W / m² (MacKay 2009).

How effectively the sun’s radiation is transmitted through the atmosphere depends on a] the frequency of the radiation and b] the radiation absorption level of the gases and vapours present, including water vapour (Twidell and Weir 2015).

If you remember, the absorption characteristics of the atmosphere are very similar to that of a glasshouse and so a greenhouse effect is experienced across the planet due to the atmosphere. This is influenced most significantly by the presence of CO₂ and water vapour in the atmosphere. The result is that sunlight arriving at the surface may be from a direct beam of sunlight or diffuse radiation. Even on a clear day it is estimated that there is always at least a 10% diffusion of photons due to molecular level scattering (ibid).

Solar energy created by nuclear fusion at the sun’s core (at temperatures of 15M°C) is released into space at temperatures of 6000°C and the rotation of the sun at these temperatures generates an intense magnetic field which affects solar activity (Smithson et al. 2008).