Numeric representation of difference between 2 line plots

Question

I have a graph with 2 plots (Plot A and Plot B, both are lines), plot A has a lower y-intercept and also lower slope than plot B. How do I represent this difference in one numeric value? Source: I am doing an experiment to compare 2 different power settings on a computer. This is a Power Vs Performance plot where lower the y-intercept and lower the slope the better. So in order to represent the difference between the 2 settings I need to come up with 1 numeric value that indicates this difference. (The most obvious would be a ratio of slopes of both the lines but that does not include the difference in y-intercepts. )