A system for determining a power requirement for a device powered by a buck converter on a system on chip based on Time on pulses (Ton) is disclosed. A buck converter supplies output voltage to enable the device. The buck converter is driven by a Ton pulse generator generating Ton pulses to control charging of a load capacitor. A counter counts the Ton pulses during the supply of output voltage while the device is enabled. A controller is coupled to the counter and the buck converter. The controller determines the power requirement for the device based on the count of the Ton pulses. The power requirement may be used to adjust the trim value to change the width of the Ton pulses for greater energy efficiency.