Heart rate and power output, unfortunately DO NOT correlate.
The dependencies are your level of 'Spriteliness' on the day.
Sleep deprevation, work or school stress, fatigue and nutrition levels contribute to a day-to-day variable HR for the same power output.
Many sessions on a gym bike will give you an idea of HR vs Power, but you MUST take notes of your Resting HR, sleep patterns, eating patterns and how many times your boss shouted at you that day.