Why not use a light sensor to turn on the light when it gets dark?
https://www.google.com/search?rlz=1C1GCEA_enMK751MK752&biw=1920&bih=938&tbm=isch&sa=1&ei=vpMHXK3GLoahsgGn7avgCQ&q=photo+sensor+light&oq=photo+sensor+light&gs_l=img.3..0l2j0i8i30j0i24l7.8649.11087..11207…0.0..0.136.1897.4j14……1….1..gws-wiz-img…….35i39j0i67.6D1l0Cxv65k