Power cannot be measured directly. Therefore, it has to be calculated from measurements of load and time, using the formula below.
Power (W) = Crank average load per single revolution (N) x 2π x crank length (m) / time required for single crank revolution (sec)
Power measurement error is determined by the accumulation of load measurement error, time measurement error, and crank length variation. In order to reduce these errors and variation, SHIMANO power meters are structurally designed based on a careful examination of crank manufacturing methods. Additionally, thorough quality control measures are applied during manufacturing and calibration. For measuring load, each individual crank is calibrated in the factory before being shipped. Crank length is established in the processing stage, with minuscule variance. For the time measurement, measurement precision is improved with the use of magnets, and at a cadence of 90 rpm, the margin of error is a minuscule ± 0.15%.