If the surface of the solar panel is glass, then you can use lacquer thinner to remove the paint. If it is plexiglass or plastic, then you can use mineral spirits paint thinner or Goo Gone. Neither one of those will harm plastic. Lacquer thinner won't harm some polycarbonates and plastics, but it will melt the surface of some types.
Dan & Ro with two Rescued Boston "Terrorists"
2015 Tiffin Allegro RED 38QRA
Previously '06 Itasca Sunrise 35A on a W22 Workhorse
'12 Jeep Wrangler Sport and/or '08 Honda Goldwing