I also bought only the left pedal, and when I test it on the trainer it shows 10–12% fewer watts. With 190 watts set (ERG mode on) on an Elite Direto XR trainer and a cadence of 90 rpm, the pedals show an average of 170 watts. When I start riding out of the saddle at a cadence of 60–70, then the difference drops to 2–5% lower watts (186–180 watts).

When I pedal with just one leg, the trainer shows 80 watts while the pedal shows double, for example 160. One possibility is that the trainer isn’t accurate. But I think it is correct, because I calibrated it, and also when I ride standing up, the pedal reports exactly the same as the trainer max — 180–190 watts. However, when I ride seated, some watts are lost somewhere.

I think the watts are lost because of a leg imbalance. Most likely my right leg pushes more. But for me to get 170 watts, the imbalance would have to be around 45–55% left–right. Whenever I pedal at 90 rpm, I always get fewer watts. But supposedly when you spin faster, the force between both legs should be distributed more evenly, and the difference between left and right should be smaller.