A good stair‐climbing (SC) ability is crucial for independent living in older adults. A simple formula that estimates the mean power needed to ascend a flight of stairs in a predetermined time (i.e., total ascent duration) is easy to implement in practice, but lacks information on actual power values generated per step. The latter is possible with body‐fixed sensors. This study aimed at comparing both methodologies and investigating their sensitivity to detect age‐related differences. 318 participants (162 ♀; age 19–85 years) were tested on a 6‐step staircase and two methodologies were used to estimate mean SC power: (1) a body‐fixed sensor with automated detection of power production per step, and (2) a mathematic equation based on timed ascent duration, body mass and stair height. SC power was 210.4 W lower with formula compared to sensor, lower in women versus men and in older versus young adults (p < 0.001). The difference in SC power between sensor and formula was greater in individuals with better performance (i.e., men and young adults) (p < 0.001), indicating a ceiling effect of the formula in well‐functioning and younger individuals. Likewise, ICC's between both methodologies showed poor reliability in people aged <65 years (0.087–0.363) and moderate to good reliability in people aged ≥65 years (0.453–0.780). To conclude, participants with better SC performance are able to largely overshoot the minimal power required to ascend the stairs in a certain duration. This makes the sensor more sensitive to identify early age‐related differences compared to the formula.