Power 5v system with Solar panels and HID lighting?

I have a 600watt HID light.  I'm wondering what the formula is to figure out how much power it would put into a solar panel.
I would like to run a few 5v devices via solar panels using just the light from the HID.
Im not sure what conversions/formulas to use.
I am drawing about 120ma with 250ma spikes on the 5v devices total.
Because I already have the 600watt light doing something... And since there is still light left over and hitting the floor I figure I can use it.
Im actually TRYING to conserve energy but in your blind enviro-treehugging-pinko rabidness you didnt bother thinking of that.
You dont seriously think I would be trying to power a 5v device with a solar panel powered by a 600watt light just because I couldnt find a voltage step down and plug it in the wall?  

The real question is how much light is one of those panels expecting to output its "rated" power.  Is a function of lumens to volts?  I can measure lumens and figure out how much is hitting the solar panel.  I also read it had something to do with the light frequency or color.