An appropriate title: "macOS Mojave battery drain".
One reason (probably not the only reason) is that Mojave is much better at GPU switching, and manages to quickly and fully shut down the discrete GPU once it is no longer being used. In HS, this did not happen. The Radeon would still be drawing power even when, supposedly, only the integrated GPU was supposed to be active.
I'm typing this on battery, and currently drawing between 6-14W while running Firefox. In HS this would be between 20 and 30W.
They should check if that behavior continues after a few days, not hours after the update.
I have had a different issue that is mildly irritating. Every time I wake my computer the brightness levels on both my laptop and external monitor are cranked up to 100%. I have the auto-adjust setting turned off, but this issue persists. Other than that, I've been pleased with Mojave, especially Night Shift.
I was very sad with Sierra but now I can work with multiple VM and IntelliJ without pain in my fingers.