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

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?

• Turns out that there is something like this already available for the OLPC: wiki.laptop.org/go/Peripherals/Hand_Crank. The efficiency of a human muscle is ca. 15-25% so a total efficiency of 10% is within reason – Alexander Sep 12 '12 at 13:49

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