Compute absolute power spectrum in MATLAB
If you have the Signal Processing Toolbox, you can use pwelch. If you do >>help pwelch   There will be some other choices. Try a few. Only you have your data so only you can determine if your results are acceptable

If you don't have the Toolbox, using FFT isn't hard. It's a matter of the length of your data, window choice, overlap, and a few other parameters