This method works for me, but only because I utilise the free energy from my solar PV Panels.
I turn the temperature down to 25°C on Sunday night, and then back up to 38°C on Friday morning when the sun is shining and giving me free power.
On the other hand, I have heard people mention on here that it makes no sense to turn it down.
The energy saved, is only spent again when trying to bring the temperature back up.
I imagine the only way this would work, is if you live in a warm environment (say California), where the water temperature would take a long time to fall.
I'm sure a mathematics genius will be along shortly with an equation.