If you hook it up to a 12v source that is the voltage that your charging system is putting out, If you want to know the amp of the charging system or the "draw" you will need to get an amp gauge, however installing an amp gauge can be hard to do and sometimes will not give and accurate reading, depending on how it is installed. Install the volt gauge to the battery and it should read aprox 11-12 volts with the car off and 13-14v with the car running. This will vary with rpm and the amount of things that you have on such as lights, ect. That is the only way that I know to install one. Best to you.