PowerTap PowerCal HRM Review – Zwifting on Your Travels with an HRM Powermeter!

The PowerTap PowerCal is a bit if an odd device. A heart rate meter, which also estimates your wattage output when cycling. In such a small package, the PowerCal might be the definitive travel equipment for Zwift, but for the need of one MAJOR question, how does the power reading compare with strain gauge derived power meters? It sounds like we need a Zwift Gear Test!!!

PowerTap PowerCal HRM Review

Zwift is addictive. Let’s face it. I know it,  you know it. But how do you keep the withdrawal chills from setting in when you have to travel? There is only a small subset of people who are going to be taking their bike with them on work trips, and even less who are willing to fly with their bike, both from a financial, but also a hassle perspective when the go on holiday!

PowerTap PowerCal

The PowerTap PowerCal is a heart rate based power meter, which might just be able to save you from the horrors of a hotel gym bike, and let you continue your scheduled Zwift workouts while away from home. So without more ado, let’s unbox, the smallest power meter I have looked at yet!

Device Design

PowerTap PowerCal

First up it should be mentioned that the PowerTap PowerCal is not a new product. When initially released in 2012, the PowerCal was also available under the Cycleops brand, and you might still find listings for the Cycleops PowerCal in the deeper parts of the interwebs. Today though however it is the PowerTap PowerCal which is being distributed from sellers.

So, what is in the box? The PowerTap PowerCal itself, and end-clipped chest strap, and a slice of dead tree with the manual printed on it

PowerTap PowerCal

To be fair, we’ve a pretty standard HRM on the outside.
Powertap Powercal

On the back, we have the RIGHT/LEFT strap connectors, Bluetooth Smart logo, plus the battery door for the CR2032 user-replaceable battery. All quite reasonable

PowerTap PowerCal

Like the 4iiii Viiiiva, which is must be said the Powertap Powercal has a VERY similar in design to (I wonder if they are sourced from the same OEM manufacturer?), the is one of the slimmer HRM’s

Powertap Powercal

As mentioned above, while the strap with the PowerCal is an end connection strap, if you prefer a centre mount chest strap, it will happily work with most other straps

Powertap Powercal

There are several different PowerTap PowerCal options, basically determining what comes in the box; either Bluetooth or ANT+  and then +/-accessories. I’ve tested the Bluetooth version here. I do think that it is a shame that there is not a dual channel version available, but then we are talking about a device which started production in 2012 before dual channel devices were more the norm. Although from the perspective of travelling with Zwift, where most people will be using Zwift iOS, the single channel BLE version will be all that people will actually need.

PowerTap PowerCal

That is kind of it. I mean only so much you can talk about a heart rate strap design!


  • Battery life: about 400 hours
  • Battery: CR2032
  • Communications: BlueTooth Smart (although ANT+ is available separately)
  • Weight: 5 g (1.76 oz)
  • Dimensions: 6.5 cm x 4.5 cm (2.55 in. x 1.77 in.)
  • Operating temperature: 0–40 C (32–103 F)
  • Waterproof rating: nil

PowerTap PowerCal

The PowerTap PowerCal manual can be found here

Using the Device

Given that the PowerTap PowerCal is performing what I would expect is a complicated calculation to derive a power reading based solely on your heart rate, I was very surprised by the lack of setup. You merely connect the strap and PowerTap PowerCal, the unit switches on and broadcasts it’s ID signal.  After a few moments, the device appears when you search for it on Zwift. You select the PowerTap PowerCal in both the power meter and the heart rate boxes, and off you go
Powertap Powercal

To put it another way, I was very surprised to find there is no calibration function; there is nothing more than strapping the unit on. The reason being that PowerTap state that there is no benefit to the user in calibrating an indirect method of power calculation such as this – I must confess, I would have still liked to have given it a go, though!

So the first thing to test was making sure that the HRM function worked. Two Garmin straps to ensure consistency and both units gave identical measures heart rates within a beat or two.

With the heart rate accuracy confirmed, it comes to looking a little bit more about how the Powertap Powercal works.

Sometimes the PowerCal would seem a little… slow-witted. What I mean is that you’ll attach it, you can connect it, but there is a good 30secs at least before the device properly wakes up and gives you a heart rate and power reading. You’ll initially see this on the pairing screen. The Powertap Powercal is detected, but it appears that you are not getting an output in spite of my frantic pedalling

Powertap Powercal

When you load into the Zwift, again pedalling doesn’t seem to have a big effect initially on your rider, but then you start to move, almost as if the unit takes about 30 secs to warm up. By comparison, other HRM units “just work” when you connect them

I even considered if this delay may actually be intentional, as unlike a normal power meter, when you stop pedalling, your PowerCal derived wattage won’t stop! Well not without a cardiac arrest!! This is highlighted below. Here I have restarted Jon’s workout, but my rider set off immediately, as I had already been using the Powertap Powercal for about five mins. Note the lack of cadence as I’m not actually pedalling, but still going forwards at a happy 34kph.

Powertap Powercal

However this “warm up” appears to be too intermittent to make me think it is intentional. This lag is something you need to keep in mind, though, if you are rushing to get into a Zwift group ride about to set off, as you might find yourself stuck at the start line, pedalling and going nowhere for 30secs!

PowerTap PowerCal Powermeter Accuracy & Zwift Gear Test

Over the last few months the PowerTap C1 has been serving as one of my long-term power meter test devices, and in conjunction with the 4iiii Precision crank, both have shown themselves to be reliable power meters. Given that, and the very indirect measure of the PowerCal, I thought it would be better to spread out the other testing site. So here I’ll be comparing agains thte C1 and the Wahoo KICKRPowertap Powercal

As ever all my tests have been run on Zwift’s work out mode – “Jon’s test”. So here is the first power graph, comparing C1 and KICKR to give to very different locations of power reading. I have also added a second orange line for my HR

Powertap Powercal

We are clearly losing out on the power compared to both “full fat” power meters with the Powertap Powercal, there is also a slight lag. The lag is understandable given that your heart is going to react to the increased works from your legs, well after the work has been done. This heart rate lag can be seen above, as my rate increases just after each peak, in time with the PowerCal readings. There are also a couple of points where the PowerCal drops power, but for no clear reason, and then has a rebound peak after, I have not been able to see why this would be – I dont appear to have any cardiac issues on the trace (Never rule out the possibility YOU could have a problem, not the device when it comes to odd heart rate patterns)

Let’s zoom into the steadier parts of the graph, where I’m doing 300 watt blocks for one min. Here we can see the Powertap Powercal seems to be happier holding steady states compared to when asked to react quickly with the higher intensity sprints.

Powertap Powercal

It is quite clear on high-intensity sprint sessions, I am experiencing significant power loss with the PowerCal

Powertap Powercal

The troughs in power also have a slight delay compared to the main power meters. The upshot of all of this that the PowerTap PowerCal doesn’t appear to have any specific benefits over Zwift’s Zpower algorithm. HOWEVER it is frequently not possible to use Zwift’s Zpower in the gym, or hotel fitness suit, as you do need to be able to provide speed and cadence data for Zpower to work, otherwise you legs will spin, and you’ll go nowhere.

Powertap Powercal

On the Gym bike, you’d also need to select “Not Listed” for the trainer,

Powertap Powercal

The Powertap Powercal is not the best power meter by a long stretch, but if you are travelling and using gym equipment, it is a power meter that will allow you to rock up and still do a Zwift work out on a fixed, no frills gym bike

I’m personally would not want to go anywhere near a race using the Powertap Powercal, but think on workouts with longer lower blocks, e.g., fat burning, the PowerCal is going to be fine for travelling. Yes you may read intermittently lower on any hard sections, but averages powers seem to come out reasonably. But then I thought, I can use the Powertap Powercal on a gym bike, what else could I Zwift on, and a lightbulb blew up in my head!!

It was when I was testing out the Powertap Powercal on a gym bike, that the thought came to me, what else could I Zwift on, and a lightbulb blew up in my head!!

Powertap Powercal

Rowing on Zwift

I have wanted to try rowing on Zwift for a long time, as in 2015 long time. I have previously attempted to go rowing on Zwift, and on two different rowing machines brands as well, but with no luck. The Concept 2 rower doesnt have anywhere to easily mount a speed sensor to, and I think a gym may object very strongly if I were to attack it with a screwdriver

Powertap Powercal

I also looked over the WaterRower, with it’s  more exposed mechanicals, but again could not seem to securely place a speed sensor to as to give what even appeared to be reason speed readings. There are some people, vastly more intelligent than I, that have managed to connect Zwift to a WaterRower using a Raspebrry Pi, however that is several stages above my engineering and coding skills, and so have no gone that route

As mentioned it was half way through this review that I realised I had a powermeter, which does not require any physical connection to the rowing machine, be it either Concept2 or WaterRower!

Powertap PowerCal

I actually found the hardest part of putting Zwift on the rowing machine was where to put the iPad! With a Concept 2, the easiest option was to just drape the iPad over the Concept2 screen. The only down side is that I then cant actually see what my workout is in terms of splits on the machines screen, but thats not really an insurmountable issue! (Yes I know I need a haircut!)

Powertap PowerCal

As for the WaterRower, the location of the of the oar strap does somewhat preclude you from actually sitting anything directly on the rower, but as long as can perch your computer or iOS device near to the rower you’ll be fine.

Powertap PowerCalThe Powertap PowerCal has actually let me use my rowing machine on Zwift in the easiest way I can imagine. Ok we have seen above that the above graphs the PowerCal isnt that much better compared to Zwift’s own Zpower on the turbo, however I cant get Zpower to function with the rowing machine due to inability to connect sensor. I have always HATED doing rowing training, focusing on just keeping to a number on a screen – I’M NOT A TT RIDER!! Zwift released me from looking at numbers and a slowly moving graph whilst on the bike, and now I’m able to cross train also on Zwift without loosing my mind!  So for that alone, I’m happy to use the PowerCal, whilst keeping a casual eye on my splits


PowerTap PowerCal

I’m sorry but as a powermeter for using day to day on Zwift, I just cant recomment the PowerTap PowerCal. Which feels like a harsh thing to say, but Zwift’s own ZPower algorthm is in the same ball park as the PowerTap PowerCal, and that is what we are talking about with a heart rate derived power meter. Ball Park levels of accuracy. Yes we see the power responding with regard to effort, and I don’t object to the slight lag particularly, but the data is reading much too low on the power in the peaks to be reasonable.

The Powercal is not, in my tests anyway,  close to the accuracy of a full blown strain guage. PowerTap, to be fair, is very clear on that, but in the Zwift world, I dont think the PowerCal has a place in NORMAL usage.

But what about travelling? After all that is the initial conceit of this post, travelling with Zwift

Powertap Powercal

There isn’t another product on the market which provides such ease of access to Zwift on the move. Just attach to chest strap and go. Also the PowerCal has given me access to Zwift, even if there is not a bike, but just a rower. I have always enjoyed rowing, from a fitness perspective, but hated that screen and staring at the numbers, yet found that my grunting and panting meant I couldn’t really watch TV either. The PowerCal allows me to row using Zwift, meaning I have something much more visual to focus on and guage my efforts against, in addition to my splits. As a result, I’m probably actually going to be doing more cross training on the rower again thanks to the PowerCal.

So should you buy a Powertap Powercal? Given the choice between ZPower, and a PowerCal, for someone who already has an existing kit, i.e. HRM, speed and cadence sensors, I’d opt for ZPower, and not consider the Powertap Powercal as an upgrade. But if I was needing a HRM monitor anyway, I’d argue a cyclist looking to do cross training whilst using Zwift in addition,will get more use of out of the ability to use a PowerCal and Zwift, than an a Bluetooth HRM which has a running cadence function built in. Particularly as they can travel with Zwif then if needed or desired

So this is a difficult summation, as a piece of technology for Zwift every day, I’ll stretch and give 2/5 to the PowerCal – it is after all a functional HRM too! But for travelling with Zwift, and getting into Zwift with least equipment possible, I’m actually going to award a  “TG Recommend” – particuarlly as you are not going to find an easier way to get Zwift working with a rowing machine! How about that for a balanced view!

PowerTap PowerCal

About James Gill

A runner first, a cyclist second, and when pushed a swimmer. Working as a GP registrar, I found that I went from walking 12miles a day on the hospital corridors to sitting in a chair. Whilst at medical school, I'd run 10km in an evening just for fun, but having lost the general activity required to be a hospital SHO, consciously being active has now become more important - well until I broke myself!

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  • How much does it cost – assuming the UK based on your passport 🙂

  • Per-Christian Barholm

    If i have rollers in the charge and want to do zone one spins for an hour or two in the morning and dont have a PM on my bike, would you recommend the product?

  • John Le Marquand

    You can get a two channel Powertap Powercal Bluetooth/Ant+ Heart Rate Strap from Pro Bike Kit.