Power measurement for cyclists can be accomplished in one of two ways: directly or indirectly. By direct is meant that some physical device actually measures force, which then is subject to the power equation in physics, which is power = force / time. Indirect measurement of power means that forces against the rider and bike are somehow measured, and then complex mathematics is involved to figure out what wattage a biker is putting out. More specifically, the indirect method of estimating power relies on Newton's Fundamental Law of Physics - for every action, there is an equal an opposite reaction resulting in a net force of zero.
So to measure wattage indirectly, sensors pick up on wind speed and other variables related to the biker's forward motion. Relying on baseline data for the rider - including bike weight, rider weight, and test ride data - it is just mathematics from there to compute force, and thus power output.
Measuring power directly is done by using strain gauges in some part of the bike. The most typical places to place force-measuring apparatus are in the crankset or in the freehub of the rear wheel. As well, there was a system that measured force through the bottom bracket, but it is no longer available; and there is a new system in the works that will measure force through pedals. To read more about measuring power directly, see this article: Cycling Power Meter.
There is one additional way to measure power which is, roughly, a direct measurement, but then again is arguably indirect. That is to measure the tension and vibration in the chain and, using engineering principles and mathematics once again, to calculate the power from that data. There is no direct contact of the chain sensor, which has a strong magnet, with the chain - there is usually a prescribed 2mm to 8mm distance between the two.
There are advantages and disadvantages with each system. Using a freehub type of setup allows the rider to switch the wheel from bike to bike, assuming that there is wiring and a computer on each bike. But then, that can limit the choice of wheels between training and racing situations. Using a bottom bracket setup means that the rider cannot switch the power system from bike to bike. The indirect measurement system can be used on any bike, though that would require moving the sensors from bike to bike and recalibrating the base pickup data accurately.
So who makes all these bike power measuring systems? Here is a synopsis of the top companies.
|Powertap||Freehub||Wired or wireless; Cycleops and Saris are top makers|
|SRM||Crankset||Wired or wireless|
|iBike||Indirect||Based on wind forces against the rider and bike|