This is one of the more brutal tests I do, but the numbers it gives me are genuinely useful.
Two max efforts, three minutes then twelve, with an easy forty minutes in between to recover. Two different durations, because that's what lets you calculate two separate numbers — you can't get both from a single effort.
What the two numbers actually mean
The first is Critical Power, the power I could theoretically hold indefinitely. It's the real line between sustainable and unsustainable effort, and often more accurate than a standard FTP test. The second is W′, my anaerobic tank. Every second above Critical Power drains it, and once it's empty I can't produce that power again until I drop back down and it refills.
291 watts for three minutes, 237 for twelve. Run through the maths and that puts my Critical Power at 220 watts, with a W′ of 12.7 kilojoules.
That's my actual power ceiling, and it's the number we're building my pacing for Japan around. I ended up building my own calculator for this too, so if you want to run your own numbers, it's linked below.