Andre Jute wrote:

Every time I come back from riding my bike, the bike computer tells me,
among other useful information like my pulse rate, how high the hills
were, how fast I pedalled, what my road speed was, the temperature,
rates of descent, etc, etc, etc, much more stuff which I have my
computer print out in neat graphs. Among all this the bike computer
tells me how many watts I expended. The thing cheats of course, as it
takes a downhill or level-road ride as zero watts (you're still
expending energy). It gives peak output and an average for the ride. So
the energy expended is the length of the ride (it only ticks the clock
when the wheels are moving) multiplied by the average output. But get
this, one kilowatt-hour is 860kJ.

Think your maths is a bit faulty. One watt second is one joule so one watt
hour is 60 x 60 = 3600 joules so one kWHr is 3600kJ which is 860kCalories.

So, if you've gone for a ride that
will burst your average audiophile (middle-aged, overweight, fatarsed,
except for Patrick and me, who are ex-athletes and cyclists still) out
into heavy perspiration, say 100W average for an hour, which allows for
some extended peaks at 250W which will drive his heartbeat up to the
maximum, which does no one any good, is 0.1kW or 86kJ. In other words,
an hour's hard ride burns only 86 nutritional calories.

I think these may be wrong too. it is generally accepted that a human at
rest generates about 100W.

Ian