The same rules apply as converting spontaneous healing spells. Sunfire is a beam of pure sunlight 50 feet long plus 5 feet per class level of the sunlord. It can hit one foe and inflicts 1d6 damage for every level of the spell converted. Thus, a sunlord who converts a 4th level spell into sunfire inflicts 4d6 damage. Opponents make a reflex save for half damage, and the pure sunlight inflicts double damage upon undead.
Fire resistance applies against sunfire.
This is an extraordinary ability. The sunlord is bolstered by the power of the sun. This does not increase the number of spells per day, only the level dependant variables such as range and damage. These bonuses do not apply at night, indoors, in heavy cloud cover, or in artifical sunlight. A sunlord standing in a deeper darkness spell effect loses these bonuses until out of the conjured darkness.