Although there are whole books written on this subject, the simple answer is that a power meter facilitates accurate pacing on the bike. Power has the advantage over heart rate in that it isn’t affected by external variables, such as race-day nerves.
Having tested for your FTP (Functional Threshold Power) – the power you can sustain for an hour – zones are calculated as a percentage of this figure, which are then used for pacing. For example, depending on experience and fitness, a sprint bike leg would be ridden at 90-105% of FTP, an Olympic at 85-95% and an Ironman at 65-80%.
By tracking metrics such as 3sec power, average power and normalised power, you can ensure that you don’t make the all too common mistake of overcooking the bike, especially the first few kilometres out of T2. Normalised power is especially useful as, by factoring in variables such as surges and periods of freewheeling, it provides a more accurate reflection of the true physical demands of the ride than simple average power. This data is pure gold dust if you’re wanting to run well off the bike.