There's been a lots of talk here about battery drain, when the car is not in use, whether overnight, or sitting for several weeks. Also, there may be added accessories that unwittingly may be draining more battery than you think.
Well, there is a way to measure this. Here is a thread from the Ascent forum that I found very interesting. They have had the same problems/complaints about batteries failing prematurely. So someone there took it upon himself to actually measure the parasitic drain while the car is idle. Someone could do this for the Crosstrek as well. Thread also has some useful information on new battery research and comparisons.