How to find energy of a signal for a given range.

I have a binary sequence. I have applied IIR Butterworth bandpass filter to the signal and plot the output with respect to the index of the input sequence.The index vary from 0 to 8000. Now I want to calculate the energy of output for index range 500 to 800 and 1200 to 1500.The code for the problem is

if true
  % code
end
R = 0.992;
theta = (2*pi)/3;
b = [1 0 -1];
a = [1 -2*R*cos(theta) R^2];
u = filtfilt(b,a,x);
Y = abs(u).^2;
window = 30;
h = gausswin(2*window+1)./window;
Z = filter(h,1,Y);
P = Z/max(Z);
hold on
plot(P);
axis([0 8000 0 1.05])

Now I want to find the total energy of P for the ranges 500 to 800, 1200 to 1500 and 5400 to 6000. Is it possible? please suggest.