Plug-in hybrids come in a variety of specs and sizes, from economy cars to mid-size sedans to luxury SUVs costing $100,000 or more.

Automakers also haven't settled on how much range is enough for a plug-in hybrid. Some offer as little as 9 or 10 miles and others as much as 53 miles on electricity before they need to rely on a gas engine.

The kinds of plug-in vehicles is expanding, and buyers have a greater selection of models with long electric ranges, including the original plug-in hybrid, the Chevy Volt at 53 miles, and the Honda Clarity Plug-In Hybrid at 48 miles. Each may be useful for different buyers' purposes.

CHECK OUT: What does a "green car" mean to you? Twitter poll results

The purpose of plug-in hybrids can be two-fold: One is just to maximize the fuel-economy benefits of a hybrid, by taking as much load off the gas engine as possible when it's least efficient, such as when accelerating from a stop or driving at low speeds. Short-range plug-in hybrids such as the original Prius Plug-In or the Mercedes-Benz C350e may accomplish this.

The larger goal for most plug-in hybrids is to be able to get through a day on electricity without ever using gas, so they can plug in at night and get through the next day without using gas either. The gas engine becomes just a range extender that makes the car practical to use for long trips on vacation.

Charge times also play a role: A plug-in hybrid with a range of up to 45 or 50 miles may not need a Level 2 charger to recharge overnight. Most owners may get by just plugging into a standard 110-volt wall socket. Longer-range pure electric cars are more likely to need a higher-powered charger.

So we thought we'd ask our Twitter followers: how many electric miles are enough for a plug-in hybrid?

Corresponding to the range of the main types of electric cars on the market, the choices are: 10-15 miles, 20-30 miles, 30-40 miles, and 50 miles or more.

Click on over to our Twitter poll and let us know how far a plug-in hybrid would have to go on electricity to make a meaningful difference in your travels.

Remember, as always, that our Twitter polls are unscientific because of our relatively small sample size and the fact that our respondents are self-selected.