Last night, not having a means to charge an IPhone 6s I turned it off. The battery level indicator at that point said the remaining charge was 21%. Restarting the phone in the morning, the battery level indicator said 11%. Does turning off and starting it several hours later normally require 10% of battery?
EDIT: the phone was completely powered off. Hold down the right-side button and slide to power off. It was in that state for at 10 hours. I wouldn't expect any power usage in such a state.
One user's experience can differ from another for a range of factors (e.g. the age of your iPhone 6s battery, the number of charge cycles it's had, etc), so what is normal for you may not be normal for someone else.
An iPhone will typically use some percentage of battery over night, but how much will depend on whether it's fully shut down or just in Airplane Mode (your question doesn't really specify). If it's only in Airplane Mode, then the iPhone is not actually switched off and a number of sensors etc are still fully operational and therefore require some power.
Also, it's not clear from your question what you mean by several hours, but I think a 10% discharge over 6-8hrs is not uncommon.
You can find a lot more info by reading what Apple has to say about it's batteries.
