Power is a measurement of the work you do on the bike, and the most accurate way to measure your effort. Unlike your heart rate, which can be widely affected by physiological influences, including: stress, lack of sleep, caffeine, temperature, time of day, dehydration and other factors, power shows exactly how much work you are doing and exactly what you are getting out of your training. Power is the perfect tool to understand how you are progressing and the best indicator of fitness gained and fatigue level. Simply put, if you want to know how you're doing, check your power.
For more detailed information check out the Stages website on the link below;