Here are a few rough estimates on the Prius: 1. The total capacity of the HV battery is the work equivalent of about 1/10 of a gallon of gas. 2. If the roof and hood were covered with photovoltaics you could get the work equivalent of about 1/9 of a gallon of gas a day for an average day in the US. 3. Each "green car" symbol of 50 Wh involves recovering the work equivalent of about a tablespoon of gas. Details are at this web link.
