I fly the EMB-145 for an airline in the US and I asked this very question to one of my ground school instructors. I don't have a very mechanical mind and I'm certainly far from being an engineer, so I was curious to know why the Embraer uses a predominantly DC electrical system while other airplanes (CRJ, etc) use a predominantly AC system. He said that, in simple terms, the capacity of DC is greater than AC so that's why Embraer decided to use a DC system. I've talked to numerous pilots and they said that Embraer got the design right because while our electrical system is very complicated, from a pilot standpoint there's not much we need to deal with in the cockpit. Our plane is very automated and uses a lot of electrical power, and even the flaps on the Embraer are electric, not hydraulic. We use a small amount of AC power for the avionics, but we never touch the buttons for that system unless it's an emergency. I hope this isn't too hard to understand, but in a nutshell DC power is used for large power draws and AC is for components that don't need such a high capacity (avionics). I hope it helps!