Things start to get strange:
I've done new battery tests, here a recap:
TEST 1
MacBook Air 2018, Mojave Latest version, battery 5 cycles of charge. Brightness 50%, volume 50%, Wi-Fi on
Battery: 100% and put in standby by clicking "sleep", and lid closed.
11:30 pm 100% of battery
08:30 am 93% of battery
08:30 am 92% of battery, Wi-Fi Off Bluetooth Off
06:45 pm 88% of battery
So, even with Wi-Fi off, it consumed 3-4% of battery
TEST 2
MacBook Air 2018, Mojave Latest version, battery 7 cycles of charge. Brightness 50%, volume 50%, Wi-Fi on, Bluetooth off.
I played a 1:45 minutes long 4K video with VLC, stored in the hard disk.
5:08 pm 100% of battery
8:08 pm 56% of battery
11:08 pm 7% of battery,
00:08 1% of battery
The battery lasted about 7 hours.
TEST 3
MacBook Air 2018, Mojave Latest version, battery 8 cycles of charge. Brightness 50%, volume 50%, Wi-Fi off, Bluetooth off.
I played a 2:30 minutes long 1080p video, stored in the hard disk.
10:04 am 100% of battery
10:18 pm 2% of battery
The battery lasted about 12 hours.
TEST 4
MacBook Air 2018, Mojave Latest version, battery 9 cycles of charge. Brightness 50%, volume 50%, Wi-Fi on
Battery: 100% and put in standby by clicking "sleep", and lid closed.
11:00 pm 100% of battery
07:30 am 100% of battery
and the results #4 is the strangest result obtained so far because I've changed nothing from the previous tests (software or hardware) and the background processes were exactly the same.
The only different thing is that for the tests #1, #2 & #3 the MacBook was in the living room, 2 meters far from my router, for the test #4 it was closed in a locker in my studio, 15 meters far from the router.
I don't know what else I could try.