Here's what I understand from your question, you want to know how to calculate 'Power' of sine wave and plot it over Power Spectrum of your model.
Power of the sine wave A*sin(x), is (A^2)/2, but only when A is constant. If the amplitude is varying, then integrate the wave over one time period and divide it by the time period to get the power.
You can then use this to add/plot the corresponding value on the Power spectrum of your model.
To plot the power of Sine wave over the existing plot, use hold on and hold of.
Refer this link for more information on Hold.