Originally Posted by
tdracer
25% of what? Because I can guarantee it wasn't airframe weight, or even fuselage weight...
In Jan Roskam's classic 1985 "Airplane Design" he quotes a formula, based on number of seats and max diff, for the weight difference between windows and windowless.
For a 150-seat aircraft, this comes out at around 200 kg.
Window sizes have grown since his book was written, so the weight penalty for newer aircraft will be a bit higher than that.