The present invention relates to a method of controlling an electro-thermal heating system in a wind turbine blade, comprising measuring a supply voltage to the electro-thermal heating system, determining a duration of a variable time based enforced off period based on the measured supply voltage, and inserting the variable time based enforced off period between subsequent switching duty cycles that controls the electro-thermal heating system. The present invention also relates to a wind turbine that comprises one or more wind turbine blades wherein each wind turbine blade comprises an electro-thermal heating system and a processor adapted to perform the method.