How accurate is Strava power / calories?

What happens if you compare PowerTap power / energy to the figures calculated by a service like Strava?

No surprises here…

I’ve been putting in a lot of miles over the last few weeks on different bikes. I don’t have power on all of them, so it’s been quite interesting to see how many watts / calories a service like Strava will calculate compared with the real amount as measured at the wheel.

This downhill segment provides a particularly stark contrast:

power

Crikey, Strava is out by around 3500 – 4000% (average of 6.67W measured, 252W calculated)… not a very good result!

The picture is better when you look at the overall energy use, which makes sense as you’d hope optimising a fake power figure for cycling mainly on the flat is a much easier job. Across these six runs, Strava calculated an average of 660kJ versus 558kJ measured, which is only an error of 15.5%

Finally, what about going uphill (a short segment at 4% gradient)?

power2

In this case the calculated power is significantly lower than reality: 11.8mph for 263W (calculated) versus 11.5mph for 292W (measured), an error of 13.7% (allowing a couple of small assumptions to normalise speeds).

Beware calculated figures! Depending on the ratio of ups and downs on your ride, you don’t even know if they’re an over or an underestimate…

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