A method and a control unit for controlling a power level being drawn from a power source (26). Based on a determined rate of change of a measured voltage (1) an estimated remaining runtime is calculated. The estimated remaining runtime represents a time where specific run criteria will no longer be fulfilled if the present power level is maintained. The estimated remaining runtime is compared to a threshold value, and in case the estimated remaining runtime is shorter than the threshold value, the power drawn from the power source (26) is decreased. Thereby the runtime for the power source (26) is maximised. This is particularly useful if the power source is a limited power source, such as a battery (26), e.g. for running a compressor (27) for a movable refrigeration system (24). The invention is suitable for automotive applications.