Seeing the relative lack of information about this I don't think anyone is really aware about it other than the developers working on Ubuntu for Phones at Canonical.
I'd expect that as with Ubuntu for Android there will be a way to run a terminal window (either by linking up to a PC) or being able to run it straight from the phone, in which case a user could type the command:
upower -d (or a similar equivalent for the Phone OS if there is a different one) into the terminal window to check the power consumption of the device.
Obviously this will vary slightly depending on what you're doing, but you'll probably be able to open relevant apps to "simulate" what you usually use your phone for, before testing and you'll have a relatively good estimate on how it performs.
Disclaimer: As there is no official documentation on this at the current date, this is just going by the assumption that the Ubuntu for Phones OS behaves very similarly to the Desktop, as such this is but an educated guess to attempt to answer your question.