Ride Work in Kilojoules Beside a Battery Rated in Watt-Hours
A power meter does one thing: it watches the watts you are pushing through the cranks and adds them up over time. What it reports at the end of the ride is work, printed in kilojoules. Wheel an e-bike out of the same garage and every number on it is in watt-hours — 400, 500, 630 on the down tube, and a range estimate built on watt-hours per kilometre. Same physical quantity, two conventions that never meet, and the moment you want to know how a hard ride compares with what a battery carries you need the factor between them.
What Each Number Is Actually Counting
Average Watts Times Hours Is Already Watt-Hours
A Rider Outputs About a Third of a Battery
One Figure Is Mechanical, the Other Electrical
Putting a Head-Unit Total Next to a Battery Label
Most of the work is deciding which number you actually have — a ride total, an interval, or a capacity printed on a pack — before either field gets touched.
Type the ride's kilojoule total straight in
Take it from the summary screen or the ride file — the field marked work, or occasionally energy. The watt-hour column resolves as you type, so a 2 347 kJ ride shows up as 652 Wh before you have finished entering the last digit.
Get a battery onto the same scale first
Pack labels give volts and amp-hours rather than watt-hours, so multiply them: 36 × 14 = 504 Wh. That figure goes on the watt-hour side and comes back as kilojoules, ready to sit beside the ride total.
Flip direction when the capacity comes first
The swap arrows put watt-hours on the input side, which is the natural order when you start from a pack and want to know how many kilojoules of riding it might replace. Typing into either box does the same job without the extra click.
Divide the watt-hours by an assist rate for range
Assist consumption runs about 5 Wh/km on a low setting over flat ground and 15 Wh/km or more on full power up hills. Dividing the converted capacity by that figure gives a range estimate far closer to reality than anything printed on the box.
Rides, Batteries and the Range Each Buys
Ride rows are average power multiplied by duration; battery rows are nominal volts times amp-hours. Range assumes assist consumption between 5 and 10 Wh per kilometre.
| Ride or battery | Energy (kJ) | Energy (Wh) | What it represents |
|---|---|---|---|
| 1 h endurance at 150 W | 540 | 150 | An easy weekday hour |
| 2 h group ride at 200 W | 1 440 | 400 | A solid amateur session |
| 4 h sportive at 180 W | 2 592 | 720 | A long day in the saddle |
| 5 h gran fondo at 210 W | 3 780 | 1 050 | Racing distance |
| 36 V × 10 Ah pack | 1 296 | 360 | 36–72 km of assist |
| 36 V × 14 Ah pack | 1 814.4 | 504 | 50–101 km of assist |
| 36 V × 17.5 Ah pack | 2 268 | 630 | 63–126 km of assist |
| 48 V × 14 Ah pack | 2 419.2 | 672 | 67–134 km of assist |
Line the two halves up and the comparison is humbling in both directions. A four-hour sportive produces 720 Wh of work — more than any single pack in the list holds — yet the same rider on an e-bike would cover that distance drawing far less from the battery, because the assist only tops up what the legs already provide. A battery is not a rider's equal; it is a very patient helper.
What Speeds Up a Ride-Against-Battery Comparison
A Label in Watt-Hours and a File in Kilojoules, Side by Side
Both boxes stay live, so the down-tube figure and the head-unit figure end up on one screen in whichever unit you want to read them, with no second calculation to keep track of.
Small Interval Totals Survive the Rounding
A 30-second effort might be 9 kJ — 2.5 Wh — and eight decimal places keep it from collapsing to zero, which matters when you are adding up a session built from twenty short repeats.
Digits Without the Unit for a Range Spreadsheet
The copy control above either field, and Ctrl+C inside it, hand over the number alone — no unit, no spacing — for the cell where you divide by an assist rate to get kilometres.
Search the List When the Ride File Reports Joules
Some head units and export formats print plain joules or megajoules instead of kilojoules, and a charging log will use kilowatt-hours; each is one search away in either dropdown.
Questions From the Head Unit and the Battery Label
What does the kilojoule number on a head unit actually measure?
Mechanical work delivered to the drivetrain, and nothing else. Strain gauges in the crank, spider or pedal read the torque you apply; a magnet or accelerometer reads the cadence; multiplying the two gives instantaneous watts, and the unit integrates that over every second of the ride. The result is force through distance, so it lands in joules and gets shown in thousands of them. Coasting contributes nothing. Nor does grinding uphill out of the saddle contribute more than the same watts on the flat — the meter cannot see effort, only output. That narrowness is exactly what makes the figure trustworthy: two rides with the same kilojoule total did the same measurable amount of work, whatever else differed.
How does a ride's kilojoule total compare with an e-bike battery?
Closer than most people expect, once the units line up. A typical 500 Wh pack is 1 800 kJ, which a fit rider matches in about two and a half hours at 200 W. The comparison flatters the rider slightly, though, because the two numbers sit at different points in the chain. The battery figure is electrical energy before the controller, motor and transmission have taken their cut — at around 80 % overall, 500 Wh stored becomes roughly 400 Wh at the rear wheel. The rider's kilojoules are already at the crank, past every loss that biology imposes. Judged on what actually reaches the road, a full battery and a long ride are remarkably similar quantities of energy, delivered on very different terms.
How far does a 500 Wh battery go at a given assist level?
Divide capacity by consumption in watt-hours per kilometre. At an economical 5 Wh/km — low assist, flat roads, a rider contributing properly — 500 Wh covers 100 km. At 10 Wh/km, which is a fair average for mixed terrain on a medium setting, it is 50 km. On full assist up sustained climbs, or with a cargo load and a headwind, consumption passes 20 Wh/km and the same pack manages 25 km. Rider weight, tyre pressure, temperature and how much the motor is asked to do from a standstill all move the number, which is why manufacturer range claims span such wide brackets. Working from watt-hours per kilometre measured on your own commute beats any published figure.
Why is a 36 V 14 Ah battery 504 Wh rather than 14 Wh?
Because amp-hours count charge, not energy, and charge only becomes energy once you know the voltage it sits at. An amp-hour is one amp flowing for one hour; multiply it by the pack's nominal volts and you get watt-hours: 36 × 14 = 504 Wh, which is 1 814.4 kJ. Miss the multiplication and two packs of identical energy look wildly different — a 48 V 10.5 Ah pack is also 504 Wh, despite showing a smaller amp-hour figure. Nominal voltage is itself an average: a 36 V pack is ten cells in series, running from about 42 V full to 30 V empty. Watt-hours are the only number that lets two batteries be compared honestly, which is why legal limits and airline rules are written in them.
Why can two riders with the same kilojoule total have had very different rides?
Because the total says nothing about how the work was distributed in time. Four hours at 150 W and two hours alternating between 400 W and soft-pedalling can both produce 2 160 kJ, yet they are completely different sessions physiologically. Normalised power exists to capture that: it weights hard efforts far more heavily than easy ones, so the second ride returns a much higher figure than its 150 W average would suggest, and training stress scores are built on top of it. Body mass matters too — the same kilojoules from a 60 kg rider and a 90 kg rider represent very different relative efforts. Work is an honest measure of output and a poor measure of difficulty.
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