Alex Dowsett recently set a new world hour record of 52.937 kilometres, how much power did he have to produce for an hour to be able to achieve this feat?
We cannot know exactly unless Dowsett's team releases it, but we can make a reasonable estimate. The most important unknown is Dowsett's drag area, or CdA, so below I have calculated the power needed to average 52.937 km/h for various values of CdA, given the assumptions noted on the figure. Notably, rho is air density in the velodrome and a trackside observer provided the estimate of 1.16 kg/m^3; Dowsett is listed as 75 kg for his "official" weight so 85 kg is an estimate of the rider plus bike plus shoes plus helmet plus suit; eta is drive train efficiency and for fixed gear bikes with a good chainline losses tend to be around 2.5%; and the coefficient of rolling resistance for track tires on a smooth wooden velodrome surface will be in the range of .0025 to .003. Thus, if Dowsett's drag area were around 0.195 m^2, his average power would have had to have been about 400 watts.
According to Canyon, it would be approximately 400 watts average. Since an hour record attempt requires that there be no computer on the bike to be official, a more accurate measurement is unlikely to be available.