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Dividing the irradiance of 1050 W/m2 by the size of the sun's disk.

Dividing the irradiance of 1050 W/m2 by the size of the sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian.

However, the radiance at the centre of the sun's disk is somewhat higher than the average over the whole disk due to limb darkening.

Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m2.

Sunlight is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy, normally from the Sun, into chemical energy that can be used to fuel the organisms' activities.