In the case of Energy CS's Electric Hybrid Prius, the engineering teams estimates that the car, if carefully driven, can get between 120-to-180 mpg; while using only 115-150 Whr per mile. The last part, Watt hours per mile, is important. Here's why.
The 9kWh Lithium-ion pack provides enough energy to propel the car at freeway speeds for about 60 miles or so -- a really exciting improvement. At that point, the car returns to normal hybrid operation, running the gasoline engine for most of the time and getting about 50 mpg.
In effect, you didn't burn a little over one gallon of gasoline for the first 60 miles or so. Instead, you consumed something less than 9kWh of electricity. Why less than 9kWh? It's a safety and durability precaution so you won't fully discharge the battery and shorten its life. So, let's say you used 80% of the 9kWh. That's 7.2 kiloWatt hours.
Now comes the fun part. Let's say you live in a city where electricity costs you 10 cents a kilowatt hour. To travel that 60 miles, it cost you 72 cents compared to the current national average price of gasoline at about $2.20/gallon in the US (as of April 2, 2005). In effect, for the same $2.20, you could drive up to 180 miles -- on three successive days, of course -- giving you the equivalent of 180 miles per gallon.
