How does a power meter on a bike work and how can you use it?

Understanding how equipment works is both interesting and useful so I thought I would write an article explaining how bicycle power meters work and how they can be used, including some practical ideas about how to train with power if you don’t own a power meter.

So how do power meters on bikes work and why are they useful? Power meters measure the force that is applied during pedalling and convert that force to power, which is the rate of energy use. Measuring power is useful because it clearly and directly indicates how hard you are riding without being effected by environmental factors. This has many advantages over other measurements such as speed or heart rate.

Different power meters measure force at different locations on the bike with varying levels of complexity. Where the power is measured can have an impact on the value of power that is measured. For example, if power is measured at the pedals, it is a direct measurement of the power you are putting into the bike, whereas measuring power at the wheel, using a hub based power meter such as a PowerTap, will read a little lower because energy is lost in the drive chain. The difference between measuring at the pedals and the wheel is usually between 3% and 5%, so not a huge amount but something that it is worth being aware of. The most important thing is knowing how to use the information that you get from your power meter so I will go on to explain that and also consider ways in which you can train with power if you aren’t ready to take the plunge of buying a power meter of your own.

Why is measuring power better than heart rate or speed/pace?

Power is unaffected by environmental factors such as wind, hills, temperature, etc, all things that can result in changes speed or heart rate even if you don’t change how hard you are working. Power is therefore a consistent metric that can be used to set the intensity of training sessions and when combined with time, measure training volume in an objective and repeatable way.

Training for any endurance sport is based on progressive overload of your body to induce your body to over compensate to the stresses imposed on it and get fitter for the particular task in hand. On a general level we can use power to calculate the work done in a given session, in a week, a month or any period of time by multiplying the average power over that period by the duration. This is a direct measure of how much work you have done, although it doesn’t accurately measure the physiological load, more of that later. If you try to do the same thing using distance, there is no account for weather you did hilly rides, spent more time riding into the wind or carried more kit during some rides than others. For distance to work you need to be doing the same rides, or very similar rides over each period of measurement. The situation is similar with using the total hours per week, although in my opinion, using time based training volume is much better than distance.

Heart rate takes account of how hard you are working but doesn’t account for the variability in the same way that power does. If your rides are of constant effort or don’t change in intensity very much then heart rate is almost as effective as power. However, heart rate is subject to physiological factors such as heat, levels of hydration, stress/anxiety and fatigue; if you are particularly tired your heart rate won’t go up as much as if you are rested. If you are doing a ride where your power varies quickly, such as with lots of short hills or corners, your heart rate will not change fast enough to account for the variability, see the next paragraph. Measuring heart rate and power is the best way forward and there are few if any occasions when it is possible to measure power and not heart rate.

When your rides include varied efforts is when measuring power really comes into its own. If you do a ride with lots of short hills, or where you are freewheeling and accelerating, your heart rate may not change very much but your changes in power will be measured directly. Accounting for this variability is very useful when considering the overall volume of training and also when planning and executing training sessions and although it is necessary to make certain assumptions in calculating volume, the result is more accurate and repeatable than using time, distance or heart rate.

It is becoming increasingly the norm to use a metric known as TSS (Training Stress Score) to measure training volume. TSS can be estimated using pace, heart rate or power. For cycling, pace is almost useless, heart rate can be reasonable for longer durations and power is quite good although not perfect. In all these measurements, some account of the type of riding must be considered but that is beyond the scope of this article.

How to start training with Power

There are many ways to train using power and not all of them require investment in a power meter. If you are on a tight budget or unsure of the benefits it is worth considering your options and using your money most effectively. My opinion is that if you can afford it, there is no question that buying a power meter is a good idea. If you are considering spending several £1000 on a bike, it is worth spending a little less on your bike and including a power meter in your purchase. Having said that, power is not essential and neither is a heart rate monitor, you can get very fit by working to how you feel; getting out and riding your bike is the main thing, so don’t get too hung up on equipment.

I coach some athletes that train with power but don’t have power meters and some who now have power meters but started working with power before they had power meters. Some athletes I work with only have power on one bike but train on more than one bike and some train with power but choose not to race with a power meter. There are many ways of working with power, it is a case of being creative and understanding how to get the benefits.

A simple way to start working with power is to use a Wattbike or similar indoor trainer that measures power in a gym. Many gyms have Wattbikes nowadays and if you are a member of a gym this is an easy way to do some power based sessions. If not, you can look for a gym where you don’t have to be a member and go there. Another option is that if you use a turbo trainer, many are now calibrated such that your riding speed can be converted to power. Have a look at the trainer websites or training apps such as TrainerRoad, Zwift, etc, for more details. Many turbo trainers measure power nowadays but my understanding is that the cheaper ones aren’t particularly accurate and if you’re buying an expensive one, why not buy a power meter and a cheap turbo trainer then you have power indoors and outdoors.

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Back to the point, assuming you now have a way to access power measurement for some of your sessions, you need to figure out how to use it effectively. Think about your event and how you need to develop your fitness. For example, if you are planning a long endurance ride such as a bike packing event, you will need to ride at a sustained level of effort for long periods of time, you won’t be doing short sprints. If you want to race criteriums on a short circuit, you will be making very variable levels of effort and working hard for a short period of time, usually less than an hour overall. It is can be a good idea to think about this variability in terms of training zones . These ‘training zones’ may look scary but all they are is a way of splitting things up in an almost arbitrary way to help think about how hard to work, to develop and execute a training plan that builds your fitness at the levels of intensity that you need for your event.

To get fitter, you need to do some training harder than you will need for your event and some easier. The harder, more intense training helps you get faster and able to sustain a hard effort for longer and the easier training builds your endurance and efficiency. Looking at the training zones in the table, if you were training for an endurance event you would want to build up the amount of time you can ride in zone 3 (Tempo), starting with say, 2 x 20 minutes with 5 minutes recovery between your efforts. Once you have built some fitness it would be good to spend some time doing Threshold and a few VO2max sessions. It is important to build up the intensity of your sessions gradually. The training zones are usually based on a metric called FTP (Functional Threshold Power), which is just the power you can sustain for a reasonably long time, 45 to 75 minutes. There are various ways to find out your FTP and once you have chosen a test protocol then stick with it. It isn’t particularly important which protocol you choose but it is important to be consistent with how you test. Here is a good way.

If you are using a gym bike or turbo trainer, it is best to do your higher intensity sessions using power and use heart rate or RPE (Relative Perceived Exertion), how hard you feel you are working, for your longer rides. If you have a power meter on your bike then you can use power for your longer rides and learn about pacing for your event.

Power during your event

Some people like to use power to help them during their event and some prefer not to know. Irrespective of whether you use power during your event, it can be a good idea to use power to practice event specific training rides so that you understand your limits, what if feels like to push too hard. Basically, using power as a guide to push your comfort zones and learn how your body behaves. Having this objective measurement can also help you practice fuelling strategies for longer events. If you do a long ride at a given intensity and get tired towards the end, it may be because you haven’t eaten or drunk enough so try the ride again and eat more. As a guide, eating 50 to 60 grams of carbohydrate an hour can be a good starting point. If you are doing a long, multi day event you need to work out your calorie requirements to sustain the event and using power is a great way to see how many calories you are using per hour or per day if you ride on terrain that is representative of your event.

If you are doing an event that lasts several hours and you are well trained for the event you can start by riding climbs in zone 3, around 220 to 240 Watts in the table, riding the flat sections in zone 2, around 175 Watts in the table and recovering on downhills. Because of the non-linear effect of wind resistance it is better to work harder on climbs and save energy on downhills. Try some rides with these values and see what happens, if you feel you haven’t worked hard enough, make some changes, if you can’t sustain the powers then you need to go easier. Make adjustments for longer and shorter events.

Related Questions:

What are the best power meters? Nowadays, most power meters are very accurate but there are differences in how they work. Some power meters work on one leg only and double the power of that leg. This works well if you produce the same or similar power with each leg or if you always use one sided power meters. However, if you change from using a gym bike, turbo trainer and one sided power meters and one leg is stronger than the other you will have a bit of inaccuracy. This isn’t necessarily a problem but it is something to be aware of and can make setting sessions and interpreting data a little more difficult. For most people, single sided power meters work fine. Other things to consider are whether you want to use hire bikes if you travel, or want to swap your power to different bikes, in which case, pedal based meters can be very effective. There are lots of reviews of power meters so have a look on the web and see what looks good.

How often should I test my FTP and recalculate my training zones? A good question! If you have just started training or you have started again after a period off the bike then testing regularly is a good idea. Wait a week or so until you are used to your setup and then do a test. Maybe test again in around 4 weeks and then you should be okay to stretch it out to maybe every 6 weeks. When you do a test, it is important to calibrate the meter and always try to use the same setup. If you are using a gym then this means using the same bike if possible.

Finally: remember that it isn’t the data or the gadgets that get you fit, it is going out and doing the training. Don’t get hung up on whether you have the best stuff, the best bike or can measure this or that, think about what you need to do to get the best out of yourself and do that, enjoy it and don’t stress.

I hope you enjoyed this article and found it useful. Let me know what you think, have a look at our other blog posts, our website and social media and let us know if you would like to see anything in particular.