In recent years, power meters have become more available and affordable to cyclists. Some power systems are more accurate than others and the actual numbers produced on one system are relative to that system. In other words, if you produce 200 watts at a given metabolic rate on one system that number may or may not match the same metabolic rate on a different power system. Accurate calibrations can help ensure accuracy from system to system, but know there are inaccuracies within systems.
A short while ago, some folks began poo-pooing heart rate monitors in favor of power meters on bikes, giving the impression that power meters are a gold-standard system not influenced by outside factors. Even the most accurate systems are affected by outside factors. The power numbers you produce on the bike, just like the heart rate numbers you produce, are influenced by heat, humidity, overall fatigue and dehydration to name a few.
Similar to using a clock for timing running or swimming intervals, a power meter can be an excellent tool to gauge pace, or more accurately, a rate of mechanical energy conversion. In a simplified view and assuming calibrated systems, the person that can produce the highest wattage per kilogram of body, plus bike weight will be the fastest rider.
If you decide to train with power, notice how your heart rate responds to the power-based intervals and how you feel. Combining heart rate, power and rating of perceived exertion is the best of all worlds. There are times you will notice that the three numbers may not match past performances for a given situation. If this happens, evaluate what might cause this to happen. Know that your fitness, life stresses, weather, course profile, current fatigue levels and other items affect the numbers.
Take away points
If you are producing 200 watts on your power meter, you may or may not be traveling faster than the person riding with you on the road, producing 200 watts on his or her power meter.
Power meters need to be calibrated on a regular basis by the owner and on occasion by the manufacturer.
Power output is affected by many of the same items that affect heart rate and perceived exertion numbers.
Power, heart rate, pace, speed and rating of perceived exertion are all tools. Using the tools and properly interpreting the data can help you improve speed and endurance.
No single tool is without faults.