solar would cost heaps for HID though.
need over 4 times the panel watts per appliance watts for it to be self sustainable and not run batteries flat, 80w to run <20w. since depending on location you might only get a few hours of full charge from the panels each day. if you can manage 6hrs of good sun a day youll only get 6 hrs of power, minus losses, @ the watts of the panel. 6 times 4 is 24
youd need 20 80w panels to run a 400w light for 24hrs/day with a good 6hrs of sun and more to be able to charge the batts
youd also need 3-4 times the battery capacity to amp-hours used so as to have reserves for cloudy days when not much charging happens and you use the batts more.
you will also need atleast twice as many amp-hours in the batts as you intend to draw from the batts, 3days reserve 6 days batt capacity, so as to not overdischarge the batteries.
its a bit like boiling water to make steam to turn a turbine to make electricity to run a heating element to boil water.