Idle musings the other day about just how much energy there is in my phone's battery, quite a lot I surmised. But, to put it into perspective, how many litres of petrol is it equivalent to?

First off, my phone has a monster battery compared to many mobile phones, 1500 mAh at 5.7 V, here's how it breaks down in terms of energy.

1500 mAh at 5.7 V = 8.55 Wh
8.55 * 3600 = 30780 J

To put that into perspective, that's almost energy to bring 100 g of water to the boil from 25 °C - Specific heat capacity of water is 4.181 J / g / K - in an ideal loss-less World. So, not quite enough to make a cup of tea, but still a bit more than I expected.

Onto the meat of the matter, how many litres of petrol is that?

In a nutshell, not many. Petrol (gasoline if you prefer) contains about 34.6 megajoules per litre (MJ/l), so our "powerful" phone battery is equivalent to 0.9 ml, or less than a fifth of a teaspoon of petrol. Bring on the fuel cells!