Does the 2020 ADS-B -Out requirement apply to aircraft with only a wind-driven generator (electrical system)?
Part 91 is mostly clear on the subject:  


  §91.225   Automatic Dependent Surveillance-Broadcast (ADS-B) Out
  equipment and use.  
  
  (a) After January 1, 2020, and unless otherwise authorized by ATC, no
  person may operate an aircraft in Class A airspace unless the aircraft
  has equipment installed that—


More detail, that describes 1090 MHz ADS-B out equipment...  


  (b) After January 1, 2020, and unless otherwise authorized by ATC, no
  person may operate an aircraft below 18,000 feet MSL and in airspace
  described in paragraph (d) of this section unless the aircraft has
  equipment installed that—


More detail describing 1090 MHz or UAT ADS-B out...  


  (c) Operators with equipment installed with an approved deviation
  under §21.618 of this chapter also are in compliance with this
  section.


If you have this deviation, you're good.  


  (d) After January 1, 2020, and unless otherwise authorized by ATC, no
  person may operate an aircraft in the following airspace unless the
  aircraft has equipment installed that meets the requirements in
  paragraph (b) of this section:  
  
  (1) Class B and Class C airspace areas;  
  
  (2) Except as provided for in paragraph (e) of this section, within 30
  nautical miles of an airport listed in appendix D, section 1 to this
  part from the surface upward to 10,000 feet MSL;  
  
  (3) Above the ceiling and within the lateral boundaries of a Class B
  or Class C airspace area designated for an airport upward to 10,000
  feet MSL;  
  
  (4) Except as provided in paragraph (e) of this section, Class E
  airspace within the 48 contiguous states and the District of Columbia
  at and above 10,000 feet MSL, excluding the airspace at and below
  2,500 feet above the surface; and  
  
  (5) Class E airspace at and above 3,000 feet MSL over the Gulf of
  Mexico from the coastline of the United States out to 12 nautical
  miles.


That's all airspace where ADS-B is required. A summary is shown in the following graphic:  


Source: FAA  


  (e) The requirements of paragraph (b) of this section do not apply to
  any aircraft that was not originally certificated with an electrical
  system, or that has not subsequently been certified with such a system
  installed, including balloons and gliders. These aircraft may conduct
  operations without ADS-B Out in the airspace specified in paragraphs
  (d)(2) and (d)(4) of this section. Operations authorized by this
  section must be conducted—


That specifies which aircraft have an exception (with following detail on specifically what airspace they can operate in.) It has, however caused confusion as the wording is very similar to Part 91.215(b)(3) which specifies "an engine-driven electrical system" as opposed to "an electrical system".  

Subsequently the FAA has published a Legal interpretation that clarifies the intent was not to create a more stringent requirement for ADS-B Out than for transponders; specifically from the interpretation:  


  As such, we have concluded that the same aircraft excluded from the
  transponder requirement are excluded from the ADS-B Out equipage.
  Accordingly, an aircraft that subsequently has been installed with
  batteries or an electric starter would not be required to equip for
  ADS-B Out.


So it appears that your J3 is exempt and would remain so unless you installed an engine-driven electrical system.