Energy of a color image

Question

I am trying to calculate the energy of an RGB image by summing up the squares of pixel values (i.e., $\sum_{i=1}^{W}\sum_{j=1}^{H}(R^2+B^2+G^2)$). I was wondering if this is the right way to calculate the energy of a color image. I found literature on calculating the energy of a grayscale image but not for a color image. Could someone point me in the right direction? Thank you.