What's Up With Strava Watts?
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  1. #1
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    What's Up With Strava Watts?

    I'm trying to get more erious about my riding and increase my speeds. I can't afford a real watt meter, so I'm relying on Strava to tell me how hard I'm working.

    Yeah, I know, Strava, but it what I've got to work with.

    I've been keeping track of my rides in a spreadsheet since 2008 and recently threw the data into a database as well. Since 2018, I started tracking my Watts, along with standard How Far / How Fast / Active Time data. In going back to Strava to retrieve data from 2015, for various reasons a benchmark year for for me, I came across some really weird stuff.

    Make no mistake about it, my 2019 self would blow my 2015 self away and say, "That's all you got?", but yet, my maximum watts rides are all in those early years. What's up with that?

    Case in point: In June of 2015 I reached a high average output of 205 Watts over 25.8 mi, with an average speed of 13.9 MPH, and a total of 346 feet climbed. Pretty flat. Now, in Nov 2018, I rode 101.7 mi in El Tour de Tucson, at an average of 15.3 MPH, and climbed 3,714 feet along the way. My average Watts for that ride: 108.

    I don't get it.

    I know I'm not world class, that's not the point here, but I am better than I was, still getting better, but the Watts numbers don't show it. Other than suggesting getting a real Watt meter, not an option at this point, can someone offer help analyzing this data?

  2. #2
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    the virtual watts from Strava are not accurate enough to worry about. If you want to get a feel for improvement, look at speed over the same course over time. There will be variance in that data due to wind etc, but it's going to be a much better indication of improvement.
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  3. #3
    tlg
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    The Strava power is nothing more than a guesstimate. There are variables they can not account for.
    Wind is fixed to constant value in the formula. That's a HUGE extremely critical variable.
    Rolling resistance is a fixed constant value. So if you use different tires/pressures you're messing up the results.
    Air density is a fixed constant value. So if you ride at different altitudes, temperatures, or humidity, you're messing up the results.
    Drag coefficient is a fixed constant value. So if you wear different clothes, change your riding position, ride the tops vs drops, you're messing up the results.


    https://support.strava.com/hc/en-us/...lculates-Power
    Our Power Equation
    The power produced while riding is made up of several components:

    Power produced to overcome the rolling resistance of forward motion.
    Power produced to overcome wind resistance.
    Power produced to overcome the pull of gravity (in the case of climbing hills).
    Power produced to accelerate from one speed to another.
    The total power produced, P(total), is the sum of all four power components.

    P(total) = P(rolling resistance) + P(wind) + P(gravity) + P(acceleration)

    P(rolling resistance)
    The power required to overcome rolling resistance can be described by the formula P = Crr x N x v, where

    P is the power required.
    Crr is the rolling resistance coefficient. We define this based on the type of bike (road, mtb, cross) you used.
    N is the normal force of the bike and the athlete against gravity.
    v is the rider velocity.

    P(wind)
    The power required to overcome wind resistance (drag) can be described by the formula P = 0.5 x ρ x v3 x Cd x A, where
    P is the power required.
    ρ is the density of air.
    v is the rider velocity, relative to the wind.
    Cd is the drag coefficient.
    A is the the surface area of the rider facing the wind.
    Because we do not know wind speed or air density during your ride, we assume no environmental wind conditions and an outside temperature of 15C. The drag coefficient is determined by the type of bike you are riding (TT bikes have less drag than mountain bikes). We use a constant for surface area. For more information about wind drag, see
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  4. #4
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    I personally have not noticed a change in strava power calculations, but I have noticed a change elevation. Some rides I have done many times have less elevation gain than before. The terrain has not change, but the base topo that strava uses must have change. Also if you changed you weight/bike weight power will change. If you changed your recording device power might have changed. Also accelerations can play a role. The more you accelerate the more power you need. If you get up to speed and stay there it takes less power on average than to slow and get back up to speed. That said it always find it hard to compare power on different rides. Same rides different day power is not too bad, but one loop vs another? Not so accurate. Also remember wind and pack riding can have major impact on power numbers and there is no way strava can know any of that.
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  5. #5
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    All of the above is correct.

    To put it as simple as we can,


    Using your " weight " as the metric.

    It takes a certain amount of power to go a certain speed over a certain % grade per the weight you put into strava.

    If you and your bike weigh exactly 200 lbs on a ride, you will need to push 200 watts to go up a 1% grade at 16.45 mph

    If you and your bike weigh 185 lbs on a ride, you will need to push 190 watts to go up a 1% grade at 16.45 mph

    ( note the numbers on this example are not exactly accurate but are close enough to illustrate the point )

  6. #6
    Pack Fodder.
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    As others have mentioned, Strava "watts" are as about as accurate as "calories burned' calculators on the elliptical at your local gym. Take them with a very, very large grain of salt.

  7. #7
    Slowski
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    Quote Originally Posted by Alaska Mike View Post
    As others have mentioned, Strava "watts" are as about as accurate as "calories burned' calculators on the elliptical at your local gym. Take them with a very, very large grain of salt.
    I was going to comment on how you can measure calories burned if power output and duration was measured, but while calculating Joules is straight forward if you know your watts and duration, I don't understand how Strava comes up with a nearly one to one ratio for kilojoules to calories when everything that I see says it should be 4.184 kj per Calorie.

  8. #8
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    Quote Originally Posted by eboos View Post
    I was going to comment on how you can measure calories burned if power output and duration was measured, but while calculating Joules is straight forward if you know your watts and duration, I don't understand how Strava comes up with a nearly one to one ratio for kilojoules to calories when everything that I see says it should be 4.184 kj per Calorie.
    Because most humans are only about 25% efficient.

  9. #9
    I love to climb!
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    On your flat ride, you were probably pedaling the whole time. On the hilly century, you probably had a lot of 0 watt times on downhills where you coasted, bringing your average watts down a lot. Kind of the opposite of average speed: avg speed goes way down when going up a hill, but only a little up when going down the hill. The average watts you gained going up were more than reduced by the 0 watts going down.
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  10. #10
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    As time passes Strava will tweak or change their algorithm for computing estimated power.

    Also, regarding different readings, the estimated power can vary significantly between the device you use.

    There is commonly a 10-25 watt difference between my Garmin Edge 1000 and iPhone. (The most common difference currently is about 10 watts on a ride.)

    Also, some Strava estimated power as recorded by some devices is way off on wattage estimates for descents.

    More important than the true or accurate measurement of what one's real wattage is as measured by a properly calibrated PM is that the Strava data is consistent. (Same effort in same circumstances using same device should be consistently the same.)

  11. #11
    Pack Fodder.
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    Quote Originally Posted by eboos View Post
    I was going to comment on how you can measure calories burned if power output and duration was measured, but while calculating Joules is straight forward if you know your watts and duration, I don't understand how Strava comes up with a nearly one to one ratio for kilojoules to calories when everything that I see says it should be 4.184 kj per Calorie.
    Here's a good article on that from TrainingPeaks.

    That said, my workout today was 90min long and the work was 1227kJ but TrainingPeaks indicated 958kcal burned. Not exactly 1:1 for me. Am I going to worry about it? Not really.

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