why kJ = kCal is a good estimate
Your PowerTap hubproduces a figure for total work performed in a ride – measured in kilojoules. This is the most accurate possible measurement of your energy investment outside of a lab. But how should you convert this figure into calories?
I’ve already written about the calculations required to convert wattage into calories, as this is more natural when planning part of a ride or workout (you wouldn’t express your threshold power in joule-hours, you’d express it in watts).
However, once you’ve actually done a ride or workout, your Powertap will give you a figure for total work performed, measured in kilojoules (kJ). This is by far the easiest way to work out calories consumed and by a stroke of good fortune you probably just want to use the kJ number for kCal.
Assumptions and working
The efficiency of cycling humans is between 20-25% (so your body’s many systems and inefficiencies burn 4-5J of energy for every 1J you deliver to the pedals).
This means we should divide any “measured joules” figure by 0.2 to 0.25 to get it expressed as “real joules”.
By a happy coincidence, a calorie is ~4.18 joules, so to turn the “real joules” into calories we’re about to do broadly the opposite (a joule is 0.24 of a calorie).
For a human with 24% efficiency, you can cancel the last two steps exactly, so that measured joules = real calories. Since the range is 20-25% efficiency this is an underestimate (lots of people will burn more calories than the calculation suggests, but few will burn less).
Obviously your bike is not perfectly efficient either- you will be losing a couple of percent of your power between the balls of your feet and the rear wheel, so the number of calories burned is going to be even higher than that figure.
However, for almost every purpose imaginable, you really want your kCal estimate to err on the low side.
Why a low calorie estimate is a good thing
Take an hour workout averaging 200W (consuming 200W * 3600s = 720kJ)
Let’s imagine you are actually at the bottom end of human efficiency (you burn a whole 5J to create 1J of power). For you, the ride requires 865kCal.
The assumption of someone nearer the top end of the scale (burning 4.17J to create 1J of power) requiring 720kCal to complete the same ride leaves a gap of 145kCal.
If you ate aiming to replace calories burned in this workout, the worst case scenario is that you will come up 145kCal short. A pound of fat is around 3600kCal, so after twenty-five of these one hour workouts you’ll have lost a pound of bodyweight you didn’t intend.
If you are one of the very, very few people who needs to avoid a pound of fat loss after twenty five workouts then you’d be better using a more conservative estimate (such as multiplying kJ by 1.1 or 1.2) but for almost everyone the unexpected loss of this extra pound of weight will be a happy mystery.
P.S. Depending on where you live, you may have kcal on food packaging (like the UK) or just “calories”, which is always really kcal (there’s some mumbo-jumbo about Calorie = 1000 calorie, not that people capitalise like that in real life, but see Wikipedia).