All airliners I have ever known about have had battery as the first source of power. The RAT if I remember right charges the batteries which provide the power to the systems.
You obviously don't know many airliners then.
On most airliners, the RAT supplies AC power to the Emergency bus. This drives at least one TRU which supplies power to the battery bus. The Standby instruments and some other essential services are AC services driven directly from the Emergency bus.
Light piston a/c and some turboprops on the other hand, has a mainly DC system. Your experience seems to be more on that side of the weight category.