Assuming about 80 watts of charging (panel wattage is rated for a higher voltage than you actually use them at) and five solar hours per day, that's about 400 watthours/day of charging, or about 17 watts average 24/7. Assuming that converter pulls about 8 watts idling you've got maybe 9 left over for your load.
Until you get another panel up you should be careful about leaving that light on all the time - you could be getting ahead of your panel and running on battery drain.
Nevertheless: Hooray! (And you've got your baseline overhead drain covered so more panels should be pure load-gravy.) Meanwhile you should be able to pull maybe 40-50 watts for five hours/day for years. B-)