Peel and rinse the sweet potatoes. Slice into thin, even strips.
In a bowl, combine rice flour, sugar, baking powder, and salt. Add ¾ cup water and mix until smooth.Add more water as needed (up to 1 cup) until the batter is thin but not watery.
Dip the sweet potatoes in the batter, letting excess drip off.
Heat oil over medium to medium-high heat until hot.
Fry in batches until golden and crisp.
Transfer to a plate lined with paper towels. Sprinkle sugar, if you like.Serve warm.
Notes
Sweet potato: Slice into thin sticks; Japanese or Korean varieties hold shape and crisp well.
Rice flour: For a light, crisp coating (use regular rice flour, not glutinous).
Baking powder: Keeps the coating light and crispy.