Solar Uniting Neighborhoods (SUN) Act of 2013 - Amends the Internal Revenue Code to: (1) expand the definitions of "qualified solar electric property expenditure" and "qualified solar water heating property expenditure" to allow a residential energy efficient property tax credit for solar energy property which is either installed in a taxpayer's residence or is located within 50 miles of such residence; and (2) exclude from gross income, for income tax purposes, gain from the sale or exchange of electricity generated by solar energy property eligible for such tax credit.