On SailfishX I've never seen more than 3-4% battery usage overnight (most times is 1-2% only), even with 7-8 open apps and an automated nightly rsync backup at 3:40am. On Jolla1 it was basically the same.
BTW I don't have yet installed any Android app on SailfishX.

Apple and Android phones were born as "monotasking" instead of multitasking because many stupid developers wasted CPU resources like there was no tomorrow and it was hard to explain to customers that their preferred apps kept wasting battery milliamperes like hungry piranhas surrounding a steak. A very few Android/iOS apps are allowed to do something when backgrounded (and only a few things like GPS updating, audio playing, network messaging, etc).

Monotasking automatically led developers to write even worse software - wasting CPU resources everytime they can - because they know that every time the app is not in foreground it's basically frozen.

Sailfish is a true multitasking system (I clearly hear Apple fanbois woohooing). Sailfish doesn't enforce memory/network/CPU/background limits, thus an app could actually waste 100% CPU cycles even in background.

Native Sailfish apps have to be well-behaved. Currently it's easy to spot a new app wasting resources for no reason... because most of native apps are actually well-behaved (thanks also to Qt/QML).

With the exception of old batteries (many hundreds of recharge cycles), if Sailfish wastes too many milliamperes then most probably you have to blame some Android app.