What is call 'energy' in matlab SMOOTHDATA help ?

I try to figure out what is the energy of input data for the smoothdata function : 'When the window size for the smoothing method is not specified, smoothdata computes a default window size based on a heuristic. For a smoothing factor τ, the heuristic estimates a moving average window size that attenuates approximately 100*τ percent of the energy of the input data.' Thanks in advance, Geoffrey

[Geoffrey Schivre]

I may have find the answer of my own question energy should be define as : E = integral s(x)^2 dx