Petrol is a highly concentrated energy source, it is also relatively cheap. Drive into a garage and the petrol pump will fill up the tank at a rate of about 1 litre per second, one litre of petrol yields 34,000,000 joules of energy when it is burnt. A petrol pump could therfore produce supply energy at a rate of 34 million watts (34 mega watts or 34MW). This is an huge amount of energy, a typical coal-fired power station might only supply at a rate of 2000MW - this is the equivalent of roughly 60 petrol pumps!