How calories convert to watts on a bike

This article explains how the number of watts you produce on the bike is connected to how much energy you consume—and how many carbohydrates you need to take in. The main question is simple: how are calories converted into power at the pedals? Through the energy principle and the concept of efficiency, we show why the body only converts a portion of chemical energy into movement and why each extra watt costs significantly more energy than you might initially think. The conclusion is clear: the more power you want to produce, the more energy you need to supply—otherwise, you risk bonking.

Before I dive in, the principle of energy needs to be laid out so that we're on the same page. It's a fundamental rule of physics that states:

Energy can neither be destroyed nor created, only transformed between different forms.

This means that when we talk about energy production, it's actually a transformation of different forms of energy.

Chemical energy from the food you consume is transformed into kinetic energy that gets you running, cycling, skiing, etc. When you cycle, we measure the work produced with the SI unit of watts.

Watt is a measure of your power, representing energy (work) per unit of time. Power (Watt) is the amount of energy transformed per unit of time and is calculated using the formula W = J/s, where J = Joule and s = seconds. Producing 1 watt requires 1 joule every second. Producing one watt for one second then consumes 1 joule, and joules can be converted to calories, and thus also to grams of carbohydrates.

Some handy values to know (it's actually about kilocalories and kilojoules, but we'll use everyday language here for simplicity):

• 1 Calorie = 4.18 Joules
• 1 Joule = 0.24 Calories
• 1g Carbohydrate (glucose) = 4 Calories

Let's show an example: 

We've established that 4.184 kJ is the same as 1 kcal. Since we can convert watts to kJ and kcal, we use the formula:

J = W × s, which also gives: kJ = W × s / 1000

Rearranging the formula shows us it costs about 0.86 kcal per hour to produce 1 watt—assuming 100% efficiency. So: if you want to produce 100 watts, it costs 86 kcal/hour; 200 watts costs 172 kcal/hour; 250 watts costs 215 kcal/hour, and so on. This applies in a combustion chamber with 100% efficiency — but our body is not a perfect combustion chamber...

There's a great calculator for this here.

The equation holds — the laws of physics apply, and energy doesn't disappear; it just changes form. However, as a cyclist, your efficiency isn't 100% where all chemical energy is converted into kinetic energy that moves you forward. In other words, not every kcal you consume goes directly through your body into your muscles and converts into 100% power at the pedals. Instead, we have an efficiency of about 20–25% due to factors like heat generation and drivetrain losses (similar to a gasoline engine).

This is where the equation starts to differ between individuals. Efficiency is a less glamorous term for our movement economy (whether movement economy is glamorous is another discussion).

Generating 200 watts at the pedals for an hour doesn't cost 172 kcal but instead 700–850 kcal (172 / 0.20–0.25). The efficiency is only 20–25% because some energy transforms into other forms rather than kinetic energy, such as heat, drivetrain losses, and potential energy (yes, there's sometimes an uphill — the more you climb, the more potential energy builds up).

For every kJ you produce with the pedals, it costs you 4–5 kJ (20–25% efficiency) where a more skilled cyclist often has slightly better efficiency. From this, we can also deduce that each additional watt you generate costs nearly 1 kcal extra per hour.

When you put aside the differences in efficiency between fat and carbohydrates and focus on the energy value of what we consume, you'll quickly see that more energy in likely means more energy out (produced watts).

Suppose you ride on the trainer at home for an hour at 200 watts. You're riding at around 85% of your max heart rate, where the body burns approximately 85–90% carbohydrates and 10–15% fat. You'll burn about 1000 kcal during this hour (as there is also a basal energy consumption beyond the activity), with about 900 of these calories coming from carbohydrates that you've stored and/or are ingesting, which totals around 225 g approximately.

To calculate your individual distribution, use the Energy Calculator.

You might recall that it costs 0.86 kcal/hour to produce 1 watt if the efficiency is 100%. With 20–25% efficiency, it costs an additional 3.4–4.3 kcal each hour for every extra watt you produce. This is one of the reasons we see individual variations and why some hit the wall sooner than others. There are slight differences in efficiency between cyclists. Additionally, there are physiological differences that allow more trained individuals to utilize more fat as fuel at the same produced watts — though not dramatically faster: at the same relative intensity (percentage of max heart rate), both Chris Froome and you burn roughly the same amount of carbohydrates and fat, even if the absolute numbers may vary.

The equation works well as long as you remain stationary on the trainer and don't get too hot. If you're outside and make a five-minute sprint uphill, some of the energy is used as potential energy since you're moving upward — it costs more energy. To conduct complete calculations for that, we'd need to involve a physicist, which could be a future article.

You can convert watts to calories burned and vice versa. It costs about 4 kcal per hour for every extra watt you want to produce on the bike. So, the harder you push, the more energy you need to have available. This is where carb loading, sports drinks, gels, bars, and other easily digestible energy forms come into play (your fat reserves go a long way, so you don't need to worry about them as much).

During multi-day races and the like, fat intake also becomes important to consume enough total calories and avoid performance drop. But this only applies to multi-day races—don't start focusing on fatty foods before you've secured the weak link in the chain: your glycogen stores.

Last but not least. Can you power a toaster that requires 700 watts? That's 2400 kcal/hour at 25% efficiency 😉