When training apps were in their infancy, our friends over at TrainerRoad made power training accessible for the masses with Virtual Power. Every trainer has a resistance curve, meaning it takes a certain amount of watts to spin the drum at a particular speed. This is a measurable and repeatable metric, and all you need to use this feature is a speed and cadence sensor.
It’s not a perfect measure because of variables like tire pressure, how snug the drum is on your tire and if the tire is warm can affect the reading slightly. Still, for those looking to get into power training, Virtual Power provides a low barrier to entry and gets you within the wattage ballpark.