From a big picture, when looking at Watts/kg it provides the most value for giving relative climbing capacity. For riding on the flats, it’s watts/CdA (aerodynamic drag) that matters the most. Watts/rider weight is the simplest way to keep things relative from person to person (just like VO2 max can be expressed as Liters/minute or milliliters/kg/min). That being said, actual climbing speed is related to not just rider watts/kg, but rider + bike + clothing/etc. system mass (kg) versus the watts that the rider puts out. Even with that, two individuals could have the exact same system mass (kg) and put out the same watts, but still go different average speed/time on the climb due to pacing strategy (see: From the Coaches: Pacing for Performance) as well as variations in the wheel mass in one equal system mass versus a different wheel mass in another. Another practical reason in cycling that we tend to look at rider watts/kg is that there is a legal lower limit of 6.8kg for the bike mass that is required for all UCI events. Of course at 6.8kg bike is a great percentage of the rider + bike system mass for a rider that weights 50kg than it is for a rider who weighs 75kg, but the bike mass still has an absolute limit. Hope that helps!
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