There seems to be an issue with Mekanism's energy usage in the electrolytic separator. The energy consumption of the machine is the same regardless of the amount of energy upgrades in it. I'm currently on version [All the Mods 7-0.3.22] and for updates that have come out nothing mentions a fix for this.

mekanism/Mekanism#7361 (comment)
It seems energy upgrades of the electrolytic separator doesn't affect energy efficiency.

I suppose it only changes the power storage capacity of that particular machine if the mod author haven't changed this setting.

as tomy linked, this is by design and not a bug