The invention relates to a method and an apparatus for converting the energy of an energy storage (2) for operating an arc (6), wherein a power unit (3) and an input and/or output device (5) for setting the current for operating the arc (6) is used to convert energy, as well as an appropriate welding device. In order to be able to provide the user with information on the state of energy storage (2), the time remaining for operating the arc (6) using the set current is calculated depending on a determined capacity of the energy storage (2) and a value for the current set on the input and/or output device (5), and said time is displayed on a corresponding display unit (27).