The average energy we receive from the Sun is 1,366 w/m^2, and this only varies by 0.1% from the activity peak to trough of its 11 year cycle. About 9% of the energy comes from wavelengths less than 400 nm. The 200-300 nm region varies on the order of a few percent, 150-200 nm by 10-20% and shorter regions by over 50% with maximums at the cycles peak activity. Energy above 400 nm is almost constant over the cycle. Why does solar energy variation over the Sun's activity cycle vary inversely with wavelength? http://astro.ic.ac.uk/research/solar-irradiance-variation
This must be due to the fact, that majority of visible spectra is emitted by whole sun surface, which have stable size & temperature.
While short-wave UV must be emitted by hotspots & bursts, number & size of which is not constant and varies in large range.