From Hydroempire.com

The noon Sunlight in Davis, California is around 5,000 lumens per square foot. Similarly, a 1 ,OOO-Watt HPS lamp with its inner arc tube 2 feet above a table is shining about 5,000 lumens on every square foot of that table. Although this HPS lamp appears to be equal to the Sun, it is not the same because its colour spectrum is poor. The Sun has another advantage over artificial lights in that Sunlight is just as intense at all distances. This is because the powerful Sunlight has already travelled some 93 million miles (149 million kilometers) to reach Earth, so the extra few feet or metres it has to go to reach the bottom of plants do not lessen its light intensity.