What Your Power Meter Cannot Tell You About Fuel

You are planning a long ride. The power target is on the screen, and you want to know what to put in your pockets. A calculator can turn watts into a calorie figure. It is tempting to take one more step and ask how much of that energy will come from carbohydrate, then pack exactly that amount.

That last step is where the confidence runs ahead of the evidence. We encountered this while researching a cycling simulator for pace24. The question was attractive: could a rider change the inputs and see how their fuel use might change? We ended up building a smaller comparison, because the broader personal prediction needed evidence we did not have.

What the pedals can tell you

Power is the rate at which work reaches the pedals. Hold 200 watts for 90 minutes and the arithmetic gives 1,080 kilojoules of mechanical work. That is a hypothetical steady ride, not an athlete's recorded result.

Your body uses more energy than reaches the pedals. The ratio between the two is called gross efficiency. It includes total metabolic energy over the same period, so we do not add resting energy a second time. Matomäki and colleagues explain why gross efficiency and other definitions of cycling efficiency are different measurements.

Suppose, purely for illustration, that gross efficiency were 24 percent. Divide the work by 0.24 and convert kilojoules to food calories: the result is about 1,076 kilocalories. Assume 20 percent instead and the same work gives about 1,291 kilocalories. The difference comes from the assumption, not from a change in the ride.

Neither number measures your expenditure. The 24 percent is an example from our research, not a recommended setting or a universal default. An efficiency measured under one set of conditions is not automatically yours on every ride, either. Power and duration alone cannot settle that question.

Total energy is not a fuel breakdown

Even if you knew total energy expenditure exactly, the next question remains: how much came from carbohydrate and how much from fat? Both contribute, and the balance changes with the conditions of exercise.

In a study of five trained cyclists, Romijn and colleagues used tracers and gas measurements at different cycling intensities. Their measurements showed greater use of glucose and muscle glycogen as intensity rose. That tells us intensity matters; it does not provide a personal fuel recipe from watts alone.

Duration, recent diet, feeding during exercise and individual differences matter too. A 2022 analysis of data from 434 studies found substantial variation remained even after modelling several of these factors. It analysed study data, not a test of an app predicting your next ride. The useful point is the uncertainty: knowing a few inputs is not the same as measuring your fuel mix.

Nor does carbohydrate used equal carbohydrate you need to eat during the ride. Your body has stores, and intake and oxidation are different things. A calorie comparison cannot tell you when those stores will run out, how much you can absorb, or what eating plan will work for you.

The choice we made

Our research examined whether we could responsibly offer a broader physiological prediction. We did not have independent validation sufficient to claim that it predicts an individual's response. One correction to a model we examined was inaccessible to us; that is a gap in our review, not proof that the model is wrong.

We chose a narrower question: if you specify constant power, duration and an assumed gross efficiency for each of two hypothetical rides, how does the calculated energy change? The comparison is labelled simulated. It does not infer efficiency from your power history, heart rate, body mass or FTP, and it does not estimate fat or carbohydrate grams. It does not change your calendar.

We checked the arithmetic against independently calculated reference cases and tested the calculator and its app integration. Those checks tell us whether the software follows its stated equations. They do not validate the equations as a personal prediction of your physiology or prove an athlete will respond a certain way.

What to take on your next ride

Treat a calorie figure as a question with assumptions attached. Ask what was measured, what was assumed and whether the number describes total energy or actually measures fuel use. If a tool turns watts straight into exact carbohydrate grams, ask how it knows.

For planning, use a feeding approach you have practised and adjust it with experience over similar rides. A calculation can help compare two scenarios; it cannot replace the information you gather by riding, eating and noticing what works for you.

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