You’ve started zwifting… first of all, welcome! One of the first things you are going to notice as you speed around Watopia is that the name of the game is power. Everything in this virtual world runs off of those precious watts that you are able to generate as you turn the pedals. If you aren’t lucky enough to have your hands on a power meter, the kind engineers of Zwift have come up with a solution for all of us! That solution is zPower.
zPower is a calculated power value based on the speed of your bike and the resistance of the trainer you are riding on. Editor’s note: although the terms are used interchangeably by Zwifters, there is a slight difference between zPower and virtual power on Zwift. Jaden is technically using virtual power, but he uses the term zPower throughout this post. For more on this, read “Virtual Power Basics for Zwifters“.)
When I started on Zwift, I did all my training and racing using zPower, and one of the first questions that I asked myself was: “Is the power value anywhere near my true power?” The more I trained the more I wanted to be able to compare my numbers with my friends and see how I would stack up in group rides and races. I wanted to know whether the numbers I was getting for FTP, training zones, etc. were close enough to my actual power to be useful.
I am basing this article on a video I created, if you’re interested in getting the information in video format, check it out and don’t hesitate to subscribe to the channel:
Hope you enjoy the video, but in case you’re more interested in the text version of the story, here it is:
The first thing I needed to do was compare two different power data charts in the same time frame. To do that, I ran Zwift as I normally would, sending speed, cadence, and heart rate to the AppleTV to run my avatar. Separately, I used the new power meter on my bike (4iiii Precision) sending data directly to my Wahoo Bolt. After I finished the test, I was able to use ZwiftPower to compare two different power graphs based on data from Zwift (downloaded from Strava) and from my Wahoo Bolt head unit, downloaded from the Wahoo app.
When you add both of these data files into ZwiftPower’s analysis tool, you get some cool comparative information. The first thing you see is this graph (the purple line is zPower, and the light blue line is the 4iiii Power Meter):
We will get to the specific sections in the graph more closely soon, but before we get to that there are some other elements of the analysis that I wanted to highlight.
For an average power, I was very impressed with Zwift! A difference of only 6.1 watts (4.26%) averaged over the whole test was actually quite impressive, so kudos to the team for working hard and making something great. I won’t talk much about normalized power, but as you can tell in the graph it is also within about 4% as well, which I would consider very accurate for an estimation.
Maximum power is definitely a key point here, with an almost 10% discrepancy. The actual power meter read a significantly higher max power, which from what I can tell is due to the ability of a rider to push significant power into the pedals instantly, but the resulting wheel speed (what zPower is actually based on) does not increase quite as fast. If your sprint lasts for a long time, you may see the zPower number climb up to match that maximum, but if the power peaks and then drops, as mine does in this sprint test, your zPower numbers will not be able to catch up, and will not spike as high a power meter can.
To review the actual data, I divided the overall test into three segments that told three different stories about the difference between Zwift zPower and an actual power meter. Here are the three main test sections that I looked closely at:
Most of the test was run as an (attempted) steady state ride, hovering around 200W. My intention here was to see what zPower could do if the speed was constant. As you can see on the graph, both of the values are tracking pretty closely together, with the zPower consistently lower, and the difference there was by about 10-25 watts. This may seem significant, especially on the higher end of that range, but the fact that it was tracking closely even though it was low helps me know that at least for workouts you will still feel the difference between hard and easy.
The other thing to notice on the steady state portion of the test was that my actual power data is quite jumpy. I’m fairly new to this power meter data, but from what I have learned from others with much more experience is that this is normal. When you first start measuring power, it’s reasonable that your natural pedal stroke will create slight variances in power even when riding at a steady state. The reason that the Zwift value is able to be so consistent, is because they are basing theirs off of wheel speed, which is more constant.
I did some really quick interval work during the test, so I could see what the zPower readings would look like when you ramp up power, stay at that level for some period of time, and then bring it back down again. I think this result is interesting because it is the only meaningful time that the zPower value is higher than my actual power meter.
The difference in the graph partially represents a slight lag, so my power increases as soon as I put more pressure into the pedals, which makes sense, but my speed will only increase gradually, thus increasing the zPower. The interesting thing to note is that my continued high-power effort increased my speed fairly significantly, and even though I was holding a consistent power, I was still accelerating. That is why for a few seconds, my zPower will continue to rise past the top of my actual power numbers. The same small lag is noticed on the tail end of the interval, as I ramp the power back down again.
The most significant difference between the values that I saw was when I tried a sprint. I only ramped my power up to around 800 watts, but it was quick enough to see how zPower would react to the changes. As expected, since my speed increases slower than the effort I’m putting in, my real power increase is very sharp. The zPower is much more gradual.
Since I spent so little time at the power peak, it is reasonable that Zwift wouldn’t reach the same value, but the difference was about 70 watts, which represented an almost 10% difference. In a significant race-ending sprint, that wattage difference could be a big deal. Another interesting piece to note is that the decrease is much more gradual on the zPower side. When I stop putting in effort, my watts will instantly go to zero. However, and depending on your specific trainer, the flywheel you have will keep some of your momentum in the trainer and your wheel will continue to spin for some amount of time as your speed decreases.
After running this test, I was honestly very surprised at the accuracy that Zwift is able to achieve through a data-based estimation. It’s actually very impressive! I know the same accuracy results will likely not be the same for everyone (see the ‘Recommendations’ for tips on how to improve zPower accuracy) but from my testing I can say that I was impressed.
Knowing how power meters work, and how the calculations are made by zPower, can be a great tool to have when you are training on Zwift and are curious as to why you can’t put out huge wattage in a sprint as quickly as your fellow racers, or why you are able to keep a much smoother and consistent power curve than those with direct power meters are. I very much enjoyed diving into this data, and if you are interested in checking out my actual data set, I have made it public on ZwiftPower! Just keep in mind that I am still in training, so the power numbers are still those of an amateur cycling enthusiast.
There are likely a few people reading who are curious whether the results that I got are going to be similar to the results that you see. Since quite a few variables are involved, I’ll share three things that you can do to help your trainer give the most accurate zPower numbers possible (see Zwift Support for more information)
- Use a compatible trainer. The Zwift website has a list of the trainers (non-smart) that they have done extensive testing with. That data is the reason that Zwift is able to calculate an accurate power. I personally use the Giant Cyclotron Mag trainer. If you use a magnetic trainer (or any trainer with a variable resistance) Zwift will tell you which resistance value to use. Make sure to use it!
- Keep the trainer tight. When you tighten the trainer to the wheel, you want to be sure that it will not slip during the workout, because that will create inconsistent speed ratings. I do this by turning the wheel directly as a test to see if you can hear any slip. If turning the wheel manually causes that skid sound, it’s a good idea to give it another quarter-turn until it’s solid. If your wheel is slipping, Zwift will usually catch it and put a pop up in-game to alert you, and it could kill your power until you tighten it (it’s the worst if you’re in the middle of a race… so best to just make it’s tight from the beginning).
- Pump the tires to a normal PSI. This part isn’t tricky, but it is easily forgotten. Just as you would before an outdoor ride, check the PSI in your tires and make sure it is at a normal value, similar to what you would ride outside.
What About You?
Have you ever compared Zwift’s zPower estimates with a power meter? What did you conclude? Share below!