Fill a one gallon glass jar with 15 cups of water (one gallon, minus 1 cup) and place 8 tea bags inside. (Use 12 bags for medium strong tea, up to 16 for REALLY strong tea.)
Tightly screw the lid on the glass jar and it outside in direct sunlight for 3 - 5 hours. *The longer it steeps, the stronger the tea. (Keep in mind when the sun moves, you may have to move your jar.)
While tea is brewing in the sun, combine ½ cup of sugar with remaining cup of water in a small saucepan.
Bring sugar and water mixture to a simmer for 4 - 5 minutes, or until sugar is completely dissolved, stirring occasionally. Remove from heat and set aside.
When tea has reached desired strength (you can tell by color), bring it inside and remove tea bags. Add cooled sugar syrup and fresh lemon juice to tea and stir to combine.
Serve over ice in tall glasses with lemon slices, if desired.
Refrigerate remaining tea for up to 3 days.