Give this a read. Look at this section on Stand By mode.
Other requirements to enter standby are different for notebooks and desktops.
Notebooks need to be running on battery power. It also must not have any external connections (such as Ethernet, USB, Thunderbolt, SD card, displays, Bluetooth, and so on).
Desktops must have no external media mounted (such as SD cards, USB, or Thunderbolt storage devices).
9% in 18 hours might be a little high, but not awful. It would use around 1-2% and her for the first three hours, then enter stand by mode and use almost no battery power after that. Once in stand by mode, it can last up to 30 days.
Look at the quote above and make sure you don't have any of the peripherals or connections mentioned.