2 Answers. A Uno which isn't doing anything useful except being turned on will use about 50 mA of current from the power jack at 9V. If you use 3 x alkaline AA batteries (giving a nominal voltage of 4.5V) directly into the 5V pin of the Arduino then you might get 2500 mAh which would be 50 hours at 50 mA per hour.Dec 28, 2017