I am using tikz to describe a load curve which is basically a clockwise rotated sigmoidal curve:load duration curve and I need a rough curve function that comes close to it.
I remember that this is just an inverted sigmoidal curve which has a form 1/(1+exp(-a(x-c))). I thought of making x the subject but that didn't yield the curve style that I was looking for.
Your curve lacks the symmetry of the sigmoidal curve you have referenced.
You could try fitting $y=Ae^{-k_1x}+B$ to the first three hours' worth of data and $y=C-De^{k_2x}$ to the last 3.2 hours, then combining the two. This won't give a $\sinh$-like curve because $k_1 \ne k_2$.