In endurance coaching, and especially in cycling, we constantly reference Joules and Kilojoules, yet rarely pause to unpack what they really mean or why they matter. These aren’t just abstract units; they’re the language of work, energy, and performance. Understanding them is fundamental to interpreting athlete output and guiding more informed, performance-focused training.

So — what exactly is a Joule?

Joules: Measuring Work Done

At its core, a Joule (J) is a unit of energy, the amount of work required to produce one watt of power for one second. A Kilojoule (kJ) is simply 1,000 Joules, and in endurance sport, it's a practical way to quantify total work done over time.

In cycling, kilojoules are used daily to quantify session load, assess fatigue cost, and evaluate work completed with more accuracy than metrics like distance or speed.

Watts: The Rate of Energy Output

To understand why Joules matter, we have to start with Watts.

A Watt is a rate. Specifically, the rate at which energy is expended.
1 Watt = 1 Joule per second.
So, 200 Watts = 200 Joules per second.

This is why power output (Watts) is so central to endurance sports: it doesn’t just tell us how much work is being done,  it tells us how fast that energy is being burned.

When we say “an athlete averaged 200W on that climb,” we’re really saying:

They expended energy at a rate of 200 Joules per second for the duration of the effort.

Joules in Practice: A Simple Example

Let’s take a common scenario in cycling: A 2-hour ride with an average power output of 200 Watts.

Here’s how we calculate the total energy expenditure in Kilojoules:

Step 1: Convert hours to seconds
2hr × 60 × 60 = 7200 seconds

Step 2: Multiply time by average Watts
7200s × 200W = 1,440,000 Joules

Step 3: Convert to Kilojoules
1,440,000 J ÷ 1000 = 1440 kJ

That means the athlete did 1440 kJ of work over that session, a direct, quantifiable measurement of energy expended.

Why This Matters in Training

Tracking energy expenditure in kJ gives coaches and athletes a meaningful way to:

• Compare workload across sessions (even if intensity varies)

• Monitor fatigue trends and manage recovery more effectively

• Evaluate an athlete’s durability and resistance to fatigue over time

• Support fuelling decisions and energy availability where relevant

• Build a more complete picture of total training load and cost

For cyclists, this is particularly valuable: kilojoules reflect the real work done, not just effort or perception. This makes them a key component in assessing both acute load and long-term progression.

From Numbers to Insight

Joules tell you what was done. Watts tell you how fast it was done. Together, they form the foundation of performance modelling, load management, and training prescription.

While the science applies across endurance sport, it’s in cycling where these metrics have become central to daily coaching, from pacing strategies to durability profiling.

Whether you're guiding athletes through multi-stage racing or reviewing solo session data, understanding these metrics isn’t optional, it’s essential.

Turn Data Into Progress With Vekta

Vekta transforms raw energy data into coaching-ready insights — helping you manage training load, track durability, and guide smarter performance.

Start your free 30-day trial or Book a Demo