batterymigration
I got a new computer from work: Macbook Pro 2015. Opened it up and the expected battery life read 14 hours.
Used Migration Assistant to upload my old computer, a 2013 Macbook Air. Migration worked fine and the laptop was plugged in the entire time.
Now, a day later, with the laptop on idle and literally zero apps open and the screen on medium brightness and all that, the expected battery life is 4 hours.
How is this possible?
EDIT Added details.
Get a program like gfxCardStatus to monitor which graphics card is in use, and watch out for Flash or other processor-hungry apps in the background.
Everything @bmike said in his comment is correct, too.
There is detailed information available about your battery.
Click on Apple (top left)
About This Mac
More Info
System Report
Power
In the Health information it shows how many cycles you had so far. I have 142 cycles since Oct. 2012. The battery should last about 1000 cycles. If you have a problem please see Apple service.
Above you see some comparison data (from my MBA) Running @ 100% brightness, Mail, Firefox, Chrome, WIFI and more, consuming about 1200 mA. Unfortunately Apple does not show the remaining life as time, so you have to use a formula to calculate that, but it varies based on usage (mA).
Best Answer
If it is relatively clean installation of macOS, then Monomeeth's answer is a good way to rectify the situation with a battery and other hardware and the OS that are not communicating properly anymore. But that way excludes the other possibilities. Therefore I'd like to add a few small bits.
Regarding "How" it's possible:
The "time remaining" is only an estimate. You shouldn't depend on that. Orient yourself on the hard data. Hard data is the electrical data: actual Current Charge and Full Charge Capacity compared to the Design Capacity. When on battery power another reading comes into play: Discharging with… Apple removed the time remaining estimate, because it is only an inaccurate estimate and apparently too confusing or disappointing to users (this is not directed at the OP).
However, a 2015 MBP is not 'new' regarding the battery in 2017. If the battery wasn't replaced in the meantime, then it is also possible that it simply didn't age so well. That might depend on usage or non-usage, being the odd lemon etc. Comparing the specs of the battery with actual readings might give unwelcome surprises there. For a quick glance of battery values use Coconut Battery or any other such tool.
Even more likely: Migration Assistant is not infallible. It might have introduced cruft or incompatible software. When that runs in the background the machine could look idling, with "zero apps open" but is working hard behind the scene. The same effect might occur if all the software brought over with Migration Assistant is trouble free but all the data still has to be indexed by active Spotlight processes. So look into Activity Monitor and Console.app and double check that the computer is really idling.