Electric cars still make up a teeny, tiny portion of the auto market in the United States (and most other countries, to be fair), but it’s hard to imagine that won’t change as the cars’ ranges get longer and their prices get smaller. Ironically, polls show that the primary reason people pay the steep prices these cars currently demand is to save money on fuel costs.

So do they?

That depends. There are a lot of variables to consider, from when and how far you typically drive to how much gasoline and electricity cost in your area.

The first thing to know is that these cars come in a couple of different flavors:

• Battery electric vehicle (BEV): All electricity, no gas. Losing your charge is like running out of gas: You’re gonna have to walk home. Most EVs currently have a range of between 80 and 90 miles, although the Tesla Model S reaches a staggering 265 miles.
• Plug-in hybrid electric vehicle (PHEV): Much more common, this kind of car runs on electricity until it runs out, and then a gasoline engine takes over. You can find ranges of 11 miles for these cars (the Toyota Prius Plug-in Hybrid, for example), but most—like the Volkswagen GTE or the Chevy Volt—seem to hit about 30-40 miles.

Just as with gas-powered cars, you find a range of efficiencies among electric cars. The current average is 30kWh per 100 miles—and 24 mpg for gas-powered cars. Of course, gas prices are ever-changing and vary according to where you live. Electricity rates are more stable but also can be different depending on location and the time of day you’re charging.

Energy.gov gives you a tailored estimate based on the rates in your area, and the U.S. Department of Energy’s fueleconomy.gov lets you compare the efficiency of any four cars at once. Which means you can find your own bottom line, more or less, but we talked to our own Dan Phillips, energy efficiency program coordinator for POWER MOVES about his experience with a PHEV. Dan, who lives in Indianapolis and drives a 2012 Chevy Volt, estimates that he saves about $400 per year over a typical sedan—using $2/gallon for gas and $.13/kWh for his calculations.

“I’m using a lot less gasoline now,” he said. “And, although I’m using a little more electricity, I’m paying less for it because it bumped me to a new rate tier that provides a volume discount.”

Phillips gets about 40 miles from his battery each day but commutes 45 miles per day and so ends up using gas for those last few miles. Even so, he says it’s worth it for him and could be for others with similar habits.

“You have to know how far you’re driving to know whether it’s going to work financially,” he said. “And the price you’re paying for electricity does depend on when you’re plugging in. There’s quite a bit of excess electricity supply at night, which is why so many utility companies offer lower rates then.”

Thus anyone who can get away with a nighttime charge and stay within the range will see the best savings from the vehicle—and even have fun driving it.

“I know they aren’t for everybody,” Phillips said, “but you really can move pretty quickly with them, especially in sport mode. I kind of like showing that off sometimes.”