Gents, thx for your suggestions.
Cummings, unfortunatelly, for what I need, I cannot use a commercial package, thus, I'm stuck with Excel even for the non-linear regressions.
Thus, regarding power, [y=a*(X1)^b1*(X2)^b2], I used
LINEST(LN(A1:An),LN(B1:Cn),TRUE,TRUE), where y is in column A, X1...