I feel like the Microsoft responses are not forthright here.
My experience with my own Surface Pro 4 is that there is a significant drop in power if not using and charging computer every day, eg while travelling,.when in "Shutdown mode".
So when one actually needs the computer, caught out because a quarter or more of the battery has discharged. If this is not a problem, why are there so many people inquiring after this issue.
Clearly "Shutdown" is NOT shutdown in the traditional sense.
There should be an option within the shutdown menu to complete a
full shutdown.
An alternative solution is as follows:
Press Windows key & R key. Then type "shutdown /s" in the box and press enter.
Appears to be effective. Takes significantly longer to start up, but power seems to have been retained..