If your asking about Strava estimating without you having a power meter, in my experience - the strava numbers are way off,

My typical 17-18 mph ride is 180w+/- on my road bike with a power meter. My cross bike, same speed with fat tires and no power meter - strava estimates 135w for the effort. Not even in the same ballpark.

Even with my power data, for some reason Strava isn't 100% in sync with my Garmin. My last ride Garmin showed 200w avg, 215 normalized power (eliminating stops and times of 0 power) - Strava showed 168W. Sometimes max power is dead on, sometimes not.