In this question it was asked how economic it is to turn your car's engine off when waiting for a traffic light. In the accepted answer it is said that it becomes more economical to turn it off if you are waiting for more than 10-15 seconds (in a comment someone said it's more likely 9-12 seconds, but still). If I understood correctly, this number applies to turning your engine off yourself manually.
My question is: what is the break-even point when using a built-in start-stop system? How can I calculate this threshold for my own car? More specifically, I drive a Skoda Fabia Greenline and want to know how long I should keep my engine turned off before it is more economical than leaving it running when stopped.
I know that start-stop systems for non-hybrid engines only work when the motor is already warm and that such engines are usually able to restart within half a second, so if you only consider fuel a start-stop restart is more efficiently than a manual restart. But start-stop restarting still costs extra battery power. I suspect that if I only stop the car for 1 second and then restart I will have used more power than if I kept it running, but I don't know if that's really the case and if so where the break-even point is.