Power might well be the ultimate training metric. Power isn’t only for racers, but for everyone who wants to see their cycling improve.
Power, as you learned in high school, is defined as force times distance over time, and is measured in watts. The point of measuring watts is to accurately quantify the force applied the pedals. And with that number, one can also determine energy spent, in kilojoules (kj). A joule is about 22% of a calorie, which, in terms of bike riding efficiency, means that one kj is roughly equal to one kcal (we generally associate 1kcal as 1 cal). With these two measures added to the usual array of collected data, modern on-bike powermeters make the bike they’re attached to a rolling laboratory, recording and storing data on power, speed, distance, time, cadence, and heart rate.
The beauty of measuring power is that it is a number that isn’t affected by internal or external factors. One of the great things about cycling is the wealth of variables that can affect the ride. Unfortunately, those same variables contribute to making training difficult to measure.