For a homework assignment I need to write a MatLab program that plots instantaneous power through a load. It is just a simple ac voltage source V=3cos(15000t) powering a load of Z=270+471.24jΩ. Attached is the output of power that I get and It seems wrong to me. Should it be just a simple sinusoidal wave, or is what I have at all correct? If it is wrong where am I making a mistake? Thank you!!!!

My code:

trange=30/15000;
dt=trange/10000;
t=0:dt:trange;

V=3*cos(15000*t);
Z=270+471.24j;
I=V./Z;
Mag=abs(I);
Phase=angle(I);
Iinst=Mag.*cos((15000*t)+Phase);
P=Iinst.*V;
plot(t,P)

The plot:

 

I don't use Matlab but it could be the 'Phase' term.

Since the current phase [lag] is constant, why not simply use

Phase=-atan(Imag(Z)/Real(Z))

 

Just multiply V with I to get the power.