I had an email discussion today with an athlete who uses power, but told me they weren't sure of their FTP. Though it is great this athlete made the leap to use power and the data to help their training, but without a baseline point to use from which to judge the numbers, they really don't mean much.
The athlete stated to me they guessed their power was somewhere around 250-275 watts. This sounds fine, but the problem is this 25 watt range represents a difference of about 10%! That is huge when talking about FTP! An athlete who can even raise their FTP 10% in one season has either stated from a point of low fitness, or done a phenomenal job of training.
Every session being off by as much as 10% in the estimate of IF and TSS changes the entire load on the athlete, and what trends can be learned, strategies for pacing can be made, and more.
It is important to know you FTP, so don't waste time with it. Here's a simple test:
Warm-up easy for 10 mins, then do 5 mins of 15 secs fast, 45 seconds easy, then 5 mins build each minute individually, easy to fast, and the fast gets faster each minute. Then take a 2 min easy spin, and start a 20 minute time-trial, of the best power you can average for the whole 20 mins. Take 95% of the average power, and that is your estimated FTP. Cool down an easy 10-15 mins of spinning.
Do this, and the numbers you use will actually begin to make sense. Without knowing your FTP, they don't mean much.