Power meters are one of the fastest moving technologies in cycling, so CW looks at what training with power can offer

Cycling is a great sport for gadget lovers. Heart-rate monitors, GPS, smartphone training apps and power meters are all there for the taking. But among them power stands out as the ‘future’ best friend for any cyclist that wishes to train to their full potential.

As such, power, or more specifically the mighty ‘watt’ has become common currency for riding and training discussions among the growing percentage of cyclists who have invested their ‘hard-earned’ in a power measuring device.

The knock-on effect is strong, and aspirational riders, listening with intent, seek to get on the bandwagon, such that now power meters are a booming market.

Quarq have a new power meter, a Shimano specific model called the Elsa RS

However, do we actually understand fully how the magical wattage figures displayed on the screens of our various bicycle computers are generated and what they’re telling us? In many cases we suspect not, and what’s more even with an understanding of what is being measured, using that information beneficially is a different ball game again.

>>> How to ride with a power meter

As we progress through this feature, we’ll summarise what products are currently available, touch on the relative pros and cons of the various systems, talk to industry experts, leading coaches, pro riders past and present, and challenge some myths and misconceptions.


Although power has only hit the mainstream in recent years, the first commercially available portable power meters, the SRM (Schoberer Rad Messtechnik) system designed by German Ulrich Schoberer, appeared over two decades ago, in the late Eighties. Cost was a big factor in the early years, making the SRM system prohibitively expensive to most, and kept it penned within physiological test labs and the pro racing ranks for a long while.

Since those early days, as with other electronica such as computers and mobile phones, technology has made the seemingly impossible viable, and costs have fallen considerably.

The science bit – What is power?

That about wraps up what products are out there, but to explain what the data is actually representing we needed to get scientific. For that we needed expert help, and a man in a lab coat – enter Professor Louis Passfield (pictured).
Not just any old prof, Passfield is currently based at the University of Kent Centre for Sports Studies, and has a vast wealth of experience in this precise field.

He was the first scientist to work and study with SRM power meters in the UK, which included his involvement in the preparation for Boardman’s successful attempt on both the Barcelona Olympics and the Hour record. He’s also a man world renowned for his work with pro riders and national squads, so with credentials like that, who better to ask?

Here’s what we gleaned… in distilled form

The maths: power (W) = force x distance / time
Watts are the energy required to a move a mass a certain distance in a known time period. 1W = 1Nm/s in other words to move one Newton one metre in one second costs one watt of energy.

So in bike speak, the mass is you plus your bike, and the distance is the ground covered, but moving a bike is a far more complicated scenario, as its resistance to motion is far from consistent, hence the measurement of power is not taken quite so literally as the raw equation implies.

Given that power = force x distance (or displacement) / time
And, displacement / time = velocity
A simplified equation is:
power = force x velocity

And that is the key to understanding how a power meter works. It’s essentially applying that equation to a given part of the bike – be that BB axle, crank, hub, pedal axle etc.

whereby, the speed of the movement, or angular velocity is measured and multiplied by the size of the force – force having been calculated by strain gauges. With those figures in hand the energy cost is therefore quantifiable as power (watts).

Accurate measurement of this force is one of the biggest challenges power meter manufacturers face. The microscopic bending measured by the strain gauge is the key to accurate results, so the smallest details become important for instance the placement of the gauges, the quality of the gauges themselves and even the temperature of the measured material. This may help to explain why the best systems are not cheap.

What is a strain gauge?

So, we know that power meters use strain gauges, but what exactly are they?

In short, a strain gauge is a component within a balanced electrical circuit ie resistance is known and constant. As a force (or strain) is applied, two things happen – the strain gauge shape deforms and subsequently its electrical resistance alters, plus there is a piezoelectric effect (a voltage generated by pressing on or squashing electrons).

These two reactions to the applied force upset the balance of the circuit and it is this difference and the resultant electromagnetic force that is effectively being measured and converted into a quantifiable measurement of force.

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Power: the game changer

Power changed training fact. Monitoring your speed can tell you only so much about a ride, heart-rate monitors can tell you a bit more, but knowing your power output is a quantitative, repeatable way to assess how hard you are actually working. As Professor Passfield perspicaciously puts it, “enabling another layer of the onion to be peeled back in the scientific conundrum of analysing performance”.

From a coaching perspective, the flip side is that with power data you can also accurately determine the specific performance requirements of a given event/discipline. That’s almost like being given the answers to the exam questions, except of course the athlete still has to put the work in to reach the targets. But just having that predictor is hugely beneficial.

Professor Passfield stresses there is still much to be learned, and we are, in his opinion, still a way off knowing enough to be overly prescriptive with training techniques. But, he says, “a power meter provides us with a far better understanding of different events so you can be incredibly precise in working out what needs to be replicated in training”.

Passfield goes on to say: “Having access to power is one thing – but having numbers alone isn’t going to make you a better bike rider – it only adds value if you are able to use the data. Heart rate data still has its place and it’s important to use that alongside wattage. Riding to a specific power alone will tell you what work you are doing but not at what physiological cost.”

For example, one use for mapping power and heart rate simultaneously is to indicate whether the pace you’re riding is sustainable. A constant power but rapidly increasing heart rate means it’s highly likely that you will soon blow. A decreasing heart rate could mean there is scope to press on and up the power output.

Power meters give you the scope to gather data for the same ride over a period of time. It’s this aspect that Passfield believes is their biggest advantage, allowing you to be more objective with your training. In other words – you might well have got a slower time over a given training route, but if your power numbers are up, for a lower physiological cost (reduced heart rate) then it’s pretty conclusive – your training is working!

Power can also tell you when it’s time to rest. Seeing either significantly lower power outputs for a known heart rate, or higher heart rate for a known power means it’s probably time to cut a ride short and head for home.

Heart rate data still has its place, and it’s important to use alongside wattage. Riding to a specific power alone will tell you what work you are doing but not at what physiological cost.

Chris Boardman interview: power in perspective

Chris Boardman A household name in UK cycling and a man synonymous with a meticulously calculated approach to training, especially in the lead-up to his successful Hour Record attempt, working with coach Peter Keen, Chris Boardman takes us through his thoughts on power and training.

“Power meters are superb tools, but you need to be able to use the data. Before you use any measuring device, you need to define exactly what you want to achieve.

It’s a bit like using scales in losing weight – it’s a tool to help you achieve your goal. “The most important thing to remember when measuring performance is the three Ps – Power, Perception and Pulse. Numbers don’t tell you how you feel. I never raced to data – either the hour record or at a prologue.

I removed the data from sight and once the effort was completed I would then look at the numbers. I simply used the information retrospectively to back up my own perception of effort.

“People forget about perception. British Cycling coaching style changes with every athlete it coaches. You have to be careful not to over-focus on the data. Power is a big part of [modern] coaching, but the rider needs think about perception.

It’s the intangible stuff, but it’s what controls the ride. Power data needs to be used by more than one person to allow it to be used in perspective – it needs a levelling voice such as a coach. Power needs to be used in partnership.

“Power can be dangerous to a super motivated person, using it without the levelling voice can mean athletes struggle to take time off. Ninety per cent of athletes who should have performed better in a race did too much training – recovery is where you improve. Power meters can help training, but need to be used with the three Ps!”

What can a power meter do for you?

Power meters are an invaluable tool in facilitating reliable, quantitative analysis of training loads and efforts, but they are not for everyone. If you’re still left scratching your head and undecided about whether to make the leap and invest, then maybe the following balanced argument will help.

If nothing else it will at least make sure you aware of the bigger picture before you get started.


– Eliminates guesswork from gauging exercise intensity, so training becomes less haphazard.
– Quantitative tracking of fitness levels over time (when combined with other physiological data – heart rate etc) that provides conclusive evidence of training adaptations, including overtraining.
– Help to define your weaknesses.
– Monitor your effort levels (pacing) during an event.
– Train more specifically for the demands of your chosen event(s).
– Aerodynamic position testing.

Shimano Dura Ace 9000 SRM Chainset


Best suited to a rider who is interested in analysis and number crunching, otherwise it tends to end up as ‘just another thing to do’. That’s an expensive lesson to learn.

It’s not an instant gratification product. Just owning a power meter does not make you an overnight pro. It takes patience, learning and dedicated training to reap the rewards.

Self-diagnosis is not a sensible or recommended path to follow. Interpretation of data requires knowledge and experience, either yourself, or with the help of a coach. The idea of having a power meter was to remove the guesswork – remember?

You can easily get bogged down by numbers. Don’t forget the fundamentals of why you ride a bike – to enjoy it. Beware of the ‘paralysis by analysis’ syndrome where you are a slave to the numbers. Trust us, you’ll quickly start to lose your ride buddies if you do.

Power meters are highly sensitive electronic devices that need to be treated with some care to maintain accuracy and functionality. They are not fit-and-forget items. Expect some upkeep and servicing costs, which might range from simple battery replacements to more skilled procedures carried out by service centres.

Five things we recommend before you start

1) We can’t stress this enough… Have a clear idea about how you intend to use a power meter in order to get the most out of owning one. Bragging rights in the cafe or just getting one because your mates have all got them is likely to be a big waste of your money.

2) Get a fitness test so you can be conclusive about what your starting point is and also provide clear goals/targets to re-evaluate.

3) Understand training zones and the need for rides to be more specific and outcome-dependant. Be aware this may mean some rethinking of your daily rides or habitual old routes.

4) Consider a coach and following a set training plan to ensure your power meter is going to provide the best return for your investment with an upward spiralling of your wattage figures.

5) Be clear about the bike, or indeed the possibility of multiple bikes that you would like to use the power meter on. This will help steer your purchasing decision, and potentially cut down your options.


Trouble deciding to make the leap? Why not rent a power meter? It’s a good way to make 100 per cent certain it’s for you before you take the plunge. Searching online is your best bet. Cyclepowermeters.com is well established in the UK, and offers all the latest designs and configurations on monthly rental plans.

Another idea might be to split the cost with a friend, and share the use of a power meter, on a weekly or monthly basis. Food for thought at least?

  • Tim Phillips

    I love the way these things on forums always get out of hand and lead to name-calling. Grow up boys! (oh, I’m a PhD physicist too)

  • Anthony Bowles

    Tommy Voekeler won the Tour de Yorkshire using his loaf, not maths!

  • Sebastien Cosnefroy

    This article is so badly written.. “…costs one watt of energy.”: watt is a unit of power not energy!

  • Gav Z

    I’m no keyboard warrior, I’d say it to your face any time, but OK you were just joking. Fair enough. But there IS an anti-geek vibe in your country and mine. The US and UK are falling way behind the far east in hi tech. manufacturing. I worked at a national lab in the US for two years. We have talent, but there are fewer science graduates year on year, certainly in the UK.

  • Harri

    Are you really sure that the “country is going down the tubes” because of a guy making a joke or for the wolly that gets upset by a keyboard warrior (you do know in replying first time that makes you no better then me fam 😉 ) and to help you out you do know the usa produce the greatest scientists and as for engineers? probs the usa as well blud i also know the uk produce a very high level of engineers and scientists as well
    so go back to getting upset about me and my joke
    oh yeah i added no grammar to my reply just to make you more upset about this country going to the dogs
    happy Friday Gav z

  • Gav Z

    Married for 10 years with 2 kids. But yeah, I know yours is the attitude of non scientifically minded people towards the technically minded. Which is exactly why this country is going down the tubes. The Chinese are turning out vast numbers of scientists and engineers, to sell us phones (like the one in your pocket), computers and the tech. of the future. Rejoice in that, why don’t you.

  • Harri

    you guys need to get girl friends!

  • Gav Z

    I agree. It is a fairly decent article.

  • Dan

    Well, I’m not a theoretical physicist. I’m an experimentalist (and we’re talking about a measurement device), and just saying “energy expended per unit time” does nothing to explain to the general audience how these power meters actually work, which was the point of the article. I stand by my conclusion: This article was actually done pretty well, especially for a sports journalist.

  • Gav Z

    Dan, I’m a Ph.D. physicist too (really). I have a Ph.D. in theoretical nuclear physics (with computational physics). At its simplest power is related to energy, and,granted, that does not have to involve mass per se. But the SI unit of power is defined using mass (as you know), and of course we have mass energy equivalence, E = mc^2. 🙂

  • Dan

    Ok, I’m a Ph.D. physicist, really. Both sides are a bit right here. It’s true that the moving mass is not directly involved in determining power for steady motion especially. The resistances to that motion are, things like wind… which doesn’t depend on the mass at at all.

    However if we look at power from the perspective of the bike on the whole instead of the pedals (yes you can do that) then power is the applied force at the ground times the velocity (there are some definition technicalities to work around here, but it’s all ok). But velocity means by definition the center of mass motion so it’s not such a terrible linguistic mistake to talk about the force acting to move a mass. Yes some forms of power don’t even involve mass at least if viewed narrowly enough, like radiative power. However that doesn’t involve force either. Radiative power is calculated only from the fields.

    The statement did not clearly say that power depends on the amount of mass, and having dealt with quite a bit of scientific reporting, I can say that this is actually quite good and certainly much better than average. It’s amazing how many sellers of energy related products even get these things wrong, especially regarding electricity, batteries etc.

    There is a very interesting point in here for cyclists though. Weight of your bike doesn’t matter nearly as much as you think. All that energy in the moving weight helps keep it coasting longer. The energy isn’t gone and it doesn’t suck up more power during steady motion.

  • Paul Appleyard

    Yes, likely true. The simple aero drag formula has air (or ‘fluid’) density as a coefficient.

  • Barney Laurance

    Agreed. And mass matters in surprising ways. In air resistance it’s the mass of the air which matters, not the mass of the bike or the rider. I believe that’s why hour record attempts are made in uncomfortably warm veloodromes – warm air is lighter.

  • Paul Appleyard

    OK, the people writing the article don’t know physics for toffee, but mass is at the heart of the definition of the Newton (the unit). And in mechanics power is force times velocity. But OK power in its purest sense doesn’t have to involve mass at all, eg. the power in electric currents or EM waves.

    They should have just said the “energy expended per unit time”.

  • Barney Laurance

    (In steady riding on flat road power is used to overcome a combination of rolling resistance and air resistance, proportionally more of the latter at higher speeds. Neither are mentioned in the article.)

  • Barney Laurance

    I’m aware of the definition of the Newton. As you said, it’s the force needed to *accelerate* 1kg at 1m/s/s. The mass matters if you’re accelerating or climbing, but not much for cruising on the flat.

    The article implies that a Newton is a measure of mass, which is wrong.

  • Paul Appleyard

    Wrong. Mass is very much at the heart of the definition of power. The definition of the Newton (unit of Force) is force needed to accelerate 1 kilogram of mass at the rate of 1 metre per second squared. The definition of the watt is joules per second, but that is derived from Power = Force * Velocity. The SI unit equivalence for the watt is 1 kg·m^2/s^3 – you’ll note the kg in there. So really mass has everything to do with it!

    Some A level physics needed methinks.

  • Guest

    Wrong. Mass is very much at the heart of power. Definition of the Newton is

  • Barney Laurance

    I think your sentance – “Watts are the energy required to a move a mass a certain distance in a known time period.” is misleading. Watts are not energy, and its about force, not mass. Mass has very little to do with it, except when you are accelerating or climbing. Newtons measure force, not mass.

    Power is force times speed, so its how hard you are pressing the pedals multiplied by how fast the pedals move in the direction you press. If you press the pedals with a force of one newton (roughly equivalent to the weight of an apple) while moving it at one meter per second, then that’s one watt of power. Double either the speed or the force and you double the power.

    Multiply that by the time you continue and you get energy. One watt for one second is one joule.

  • Lee

    You made it sound more complicated than it really is. Power is simply a measure of how hard and fast you are pressing the pedals.