According to wikipedia: "UV radiation constitutes about 10% of the total light output of the Sun, and is thus present in sunlight" It also states that "at ground level sunlight is 44% visible light, 3% ultraviolet". Another article states that at ground level "3 to 5 percent is ultraviolet (below 400 nm)". Further, the energy emitted from the sun is about 1,400 watt per square meter (on the ground level).
Based on this data I understand how to calculated the luminous efficacy of visible light. The sun has a efficacy of 93 lumens per watt of radiant flux, which works out to about 98,000 lux of visible light per square meter.
However how do you calculate the irradiance (or radiant flux?) for the non visible spectrum (such as violet light)? In other words, while I know the number of watts per meter how do I calculate the actual amount of light (in the UVA spectrum) that reaches the ground by the sun per square meter? How much of this energy is actually converted into UVA light? Or how efficiently does the sun convert energy in the non visible spectrum?