A device (1) for providing an amount of power to a gas discharge lamp (2) comprises a control circuit (3) for controlling a supply circuit (4) for supplying the power according to a power versus voltage graph (10). A calculator (30) calculates a boundary voltage value as a function of a measured voltage value of a voltage signal that has been measured after a predefined time-interval from a cold start of the gas discharge lamp (2). A more accurate boundary voltage value results in more stability and in less time required to reach a steady state. The calculator (30) may be arranged for calculating the boundary voltage value as a function of a minimum voltage value of the voltage signal and of a steady state voltage value of the voltage signal. A memory (31) may store voltage values of the voltage signal and a processor (32) may update these voltage values.