My above answer wasn't fully accurate To clarify further:
- Watching Youtube/Netflix videos for 1hr 20 mins, battery power was consumed at ~12-15% per hour, which seems OK
- When in standby mode, screen switched off, no background apps running, and a few possible battery-consuming features/settings disabled, battery power was consumed at a rate of ~4% per hour, which is shockingly high.
- In standby mode, screen off, no background apps, but with every possible battery draining setting/feature switched off including Bluetooth, but still with WiFi enabled, consumption dropped to a very reasonable ~1/2% per hour.
You can see these three consumption gradients in this screenshot - first on-charge, then steeply sloping while watching videos for 1hr 20min, then moderately sloping for 6 hours while in standy, then almost flat for 12 hours when in standby but with "everything disabled":
To reiterate; the left/middle-ish moderately sloping, section lasting ~6 hours of the plot above corresponds to scenario 2. as described above: screen off, standby mode, no background apps running (NB 'Battery Usage by App' during such periods show no app as consuming any power), yet it drains battery in this scenario ar around 4%/hour.
What's that all about?
I've only managed to get battery consumption down to expected(acceptable!) levels in standby mode by disabling pretty much every iOS feature I can think of (except WiFi) that might possibly consume power when the iPads not being used by me. Scenario 3 above, the rightmost very slightly-sloping (almost flat) section of the plot above lasting around 12 hours. Those steps shouldn't really shouldn't be necessary in order to gain acceptable battery drain in standby (non use) mode!