I don't think it's normal. Are you sure your laptop goes into any sleep mode at all? I have 9300 and it uses about 5Wh a day when in S2 sleep and about 2Wh a day when in S3 sleep mode. Fully discharging in 24hours (assuming your battery is about 50Wh) pretty much means your laptop keeps consuming about 2W of power -- doesn't sound like sleeping to me...

I've measured - I've lost 60% battery overnight (8 hours - left the machine at 2300 and came back at 0700)
I checked the journal, those are the related events (no other events from 23:15 to 06:58):

Jan 25 23:15:35 xps systemd-sleep[4337]: Suspending system...
Jan 25 23:15:35 xps kernel: PM: suspend entry (s2idle)
Jan 25 23:15:35 xps kernel: Filesystems sync: 0.004 seconds
Jan 26 06:58:29 xps kernel: Freezing user space processes ... (elapsed 0.003 seconds) done.
Jan 26 06:58:29 xps kernel: OOM killer disabled.
Jan 26 06:58:29 xps kernel: Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
Jan 26 06:58:29 xps kernel: printk: Suspending console(s) (use no_console_suspend to debug)
Jan 26 06:58:29 xps kernel: psmouse serio1: Failed to disable mouse on isa0060/serio1
Jan 26 06:58:29 xps kernel: ACPI: EC: interrupt blocked
Jan 26 06:58:29 xps kernel: ACPI: EC: interrupt unblocked
Jan 26 06:58:29 xps kernel: OOM killer enabled.
Jan 26 06:58:29 xps kernel: Restarting tasks ... done.
Jan 26 06:58:29 xps kernel: mei_hdcp 0000:00:16.0-b638ab7e-94e2-4ea2-a552-d1c54b627f04: bound 0000:00:02.0 (ops i915_hdcp_component_ops [i915])
Jan 26 06:58:29 xps systemd[1]: Condition check resulted in Run anacron jobs being skipped.
Jan 26 06:58:29 xps systemd[1]: Condition check resulted in Daily apt upgrade and clean activities being skipped.
Jan 26 06:58:29 xps systemd[1]: Starting Message of the Day...
Jan 26 06:58:29 xps systemd[1]: Condition check resulted in Rotate log files being skipped.
Jan 26 06:58:29 xps systemd[1]: Condition check resulted in Daily man-db regeneration being skipped.
Jan 26 06:58:29 xps systemd[1]: motd-news.service: Succeeded.
Jan 26 06:58:29 xps systemd[1]: Finished Message of the Day.
Jan 26 06:58:29 xps kernel: thermal thermal_zone8: failed to read out thermal zone (-61)
Jan 26 06:58:29 xps systemd-sleep[4337]: System resumed.
Jan 26 06:58:29 xps kernel: PM: suspend exit
Jan 26 06:58:30 xps systemd[1]: systemd-suspend.service: Succeeded.
Jan 26 06:58:30 xps systemd[1]: Finished Suspend.
Jan 26 06:58:30 xps systemd[1]: Stopped target Sleep.
Jan 26 06:58:30 xps systemd[1]: Reached target Suspend.
Jan 26 06:58:30 xps systemd[1]: Stopped target Suspend.
Jan 26 06:58:30 xps systemd-logind[1128]: Operation 'sleep' finished.
Jan 26 06:58:30 xps ModemManager[1201]:   [sleep-monitor] system is resuming

  from this log it looks like it starts the suspend process at night and finishes it only when I wake it up.

Any advise, anyone?
Dell support will not take any action to assist me be because the machine was purchased with Windows rather than Ubuntu (Developer's Edition not available in my region)

Also, does anyone knows if the Ubuntu that comes preinstalled in developer edition is any different than the official public ISO?

Inspiron 5510 here, same problem. s2idle is the only possibility.
I figured that problem out because, assuming it was going to deep sleep when closing the lid for the night, the battery was completely drained the next morning. It drains the battery at an incredible amount.
I'm sorry I can't answer your questions but since the original topic is closed, and yours seems to be the only one for this year, i do my part complaining about this very annoying problem that should be fixed, and not only for the "Dell Ubuntu". It's annoying for the Linux community. If I knew before, i would have reconsidered my purchase.
Dell, please update your firmware.
Thing is, I don't have much hope but I wondered if downgrading the BIOS could fix things?