How much time can I power my laptop by eating one dessert?

Question

Random question that popped into my mind after a 4-hours power outage. Let us assume that I am eating an extra dessert (250 kcal) and that I am using a bike and a generator to power my laptop (it consumes 10W while idle). How much time can I use the laptop?

I computed this as follows: $$ t = \frac{E}{P} = \frac{250 kcal}{10W}=\frac{250 * 4,18KJ}{10J/s} = 1.045 * 10^5s=30h $$

The above calculation assumes 100% efficiency. But even if we assume 10% efficiency, I would still be able to power my laptop for $3h$ and all this by eating just one extra dessert.

Questions: Are my calculations correct? Are my assumptions reasonable? Why don't we see a proliferation of bike-laptop devices?

Answer 1

There's some discussion on this subject by AlanSE and me at How fast would someone have to run to run over water?.

A typical human i.e. not a trained athlete can manage a mechanical output of about 200W, and at this speed you burn about a 1,000 Calories per hour. Assuming it scales linearly (it probably doesn't!), and assuming 100% efficient conversion of mechanical power to electricity, powering your 10W laptop would consume about 50 Calories per hour. Your estimate of 3 three hours is in line with this given all the approximations involved.

As for why we don't see a proliferation of bike powered laptops, it's kind of hard to take the bike generator along with me on the train :-)

Answer 2

A litre of gasoline costs around \$1 and contains around 35MJ of energy. A Mars bar costs \$1 and contains 250Kcal = 1MJ Cycling at 15km/h uses about 30W so 1MJ = 9 hours of gentle cycling = 140Km

So the question is why don't we see more cycle powered cyclists!