Not sure anybody else on here uses iMac, but in case anybody is interested, I happened to notice that after "up"-grading to Mojave, mine was failing to sleep/standby, thus using a lot more electricity.
One underlying problem here is that there isn't a power led to look at, and so no easy way of telling if this 2017 iMac has actually entered any kind low power mode. The screen switches off which would fool a lot of people into thinking is was now asleep, but consumption stays unreasonably high, as CPU/memory/disks whatever seem to stay powered up. I deduced that as I leave it connected via a plug-in watt meter and found that with Mojave, even after being idle from 5pm til breakfast time, it was still consuming 25-30W. To quantify it as cash, it's going to be costing me something over £30 per year, or say £200 over its lifetime, just for sitting there looking pretty (the iMac, not me).
The GUI offers only very limited control, I had to resort to command line to figure it out. I don't think the command line parameters are officially documented, but the difference (from High Sierra)
seems to be that a parameter named "standbydelay", which was set to 10800 seconds (3 hours) by default, has been replaced by "standbydelayhigh" and "standbydelaylow", which are each set to a staggering 86400 (24 hours) by default. I think "high" and "low" are used for laptops, and refer to battery charge levels. "sudo pmset standbydelaylow 10800", and same for "...delayhigh" seems to have restored sensible behaviour, consumption reducing to less than 2Watts, about 3 hours after I walk away.
I have also disabled (from the GUI) "power nap". Not sure if that was essential, but seems sensible. Also worth mentioning that after tweaking the GUI, the "pmset" tweaks sometimes go back to their default values.
ISTR that High Sierra also had (different) problems with energy-saving modes, that involved 'pmset' tweaks to make it work properly.
