It takes water to drive your car. Not in the sense that your car is powered by water, but in the sense that our power plants in the U.S. often use steam for either cooling or to power turbines. In fact, at 195 billion gallons of water consumed per day, power generation is second only to irrigation when it comes to using our water supplies. According to a study published by the American Chemical Society, PHEV's require 17 times the water supply that gas-powered cars need. That's a huge difference, and it's a constant figure unless you are getting your power from renewable sources, like wind, geothermal or solar. Of course, that "study" needs some explanation, which CalCars provides in spades. Basically, since many of those gallons of water aren't "used up" and other forms of energy generation use water as well, the anti-EV sentiment is out of porportion. Carey King, the researcher who worked on the study called the Popular Mechanics' headline "unnecessarily alarmist" and said that "You'll see that I don't believe the water intensity is a hindrance for PHEV/EVs."
Still, issues like water use remind us that there are many considerations to make when purchasing your next vehicle. Off-peak charging of your PHEV is one way to be sure that you are not putting to great a burden on water supplies; paying extra for renewable power is another.