You have broadband noise. There may be a signal at about 0.35 Hz.
The Fourier transform analysis:
D = xlsread('power.xls');
t = D(:,1);
p = D(:,2);
L = length(t);
Ts = mean(diff(t)); % Sampling Interval (sec)
Fs = 1/Ts; % Sampling Frequency (Hz)
Fn = Fs/2; % Nyquist Frequency (Hz)
FT_p = fft(p - mean(p))/L; % Fourier Transform (Discrete)
Fv = linspace(0, 1, fix(length(t)/2)+1)*Fn; % Frequency Vector
Iv = 1:length(Fv); % Index Vector
figure(1)
plot(Fv, abs(FT_p(Iv))*2)
grid
I will leave it for you to sort this.
