The total voltage of a series circuit is the sum of the voltage of each power source connected in series. In the example circuit, there is only one power source, which is a 9V battery, therefore, the total voltage is 9 volts. If we used 4 AA batteries (1.5 volts each) in series, rather than the 9-volt battery, we would have a total voltage of 6 volts because 1.5V + 1.5V + 1.5V + 1.5V would equal 6 volts.