Is it possible to use solar lights indoors?

Yes, this can be achieved by:

  1. Placing the solar panel in a window that receives at least 3 hours sunlight
  2. Charging the unit externally and bringing it indoors to use

All you need to do is make sure the solar panel is covered/not exposed to another light source when you want the lights to shine.

Try using the lights on your Christmas tree. 
They look beautiful, are safe to touch (don’t get hot) and are guaranteed not to cause an electrical fire.