5S 8.1 battery usage.

Seems fine.

As for 18% it's simply 18% of the time that you've used the phone that it was on the home screen (locked or unlocked), not that it used up 18% of the battery power basically.

 

My wife got 5 hours usage and 24 hours standby. With around 35% battery left. iPhone 5s. I have the usual optimizations going like limited background app refresh an other stuff.

 

My 5s seems ok - no difference from older 8.x or ios 7.

I believe Home screen & lock screen usage includes notifications (I.e the banner that pops up and wakes the phone when you get a notification)

 

The battery life has worsened for my 5s under 8.1. Barely getting through the day now.

 

Home and lock screen mean the screen is using power. Whether it be when the screen is waking for notifications or just during normal usage, it falls under the 'home and lock screen" part of usage.