> See all Guides 
The gallon is a familiar measurement for most people. Just think about a gallon of milk, and you know exactly how big it is. But the kilowatt-hour—an extremely useful term for EV drivers—is unfamiliar.
Gas stations, after more than a century on the scene, have harmonized around the cost of a gallon of gas, using big letters on marquees to announce pricing. On the other hand, electric vehicle charging costs are not nearly as clear.
It’s important to gain a grasp of these pricing schemes so you can compare the cost of charging in public to what you pay at home. Using household electricity will almost always be less expensive than stations that aren’t free. But how much cheaper?
While various factors make it difficult to set the benchmark price for home charging, we can point to a national average price for a kilowatt-hour of household electricity, as documented by the U.S. Energy Information Administration. For April 2014, the EIA shows an average retail price of $0.1231 per kilowatt-hour (kWh). Of course, your price—depending on where you live, and your specific rate plan—will vary.
Pacific Gas & Electric in Northern California offers an EV rate plan, which sets the price at $0.04 per kWh when charging after midnight. That’s a third of the national average. Also, let’s not forget that the efficiency of your EV determines how far you go on a kilowatt-hour of energy. Electric cars go between two and five miles on a single kilowatt-hour. That means, the average EV driver will spend three pennies or less on electricity for every mile of driving. Some utilities, like this one in Texas , even give electricity away after midnight!
With these complexities in mind, it’s still possible to examine various public EV charging scenarios to determine if you are likely to pay more or less than your home-based electric fuel.
The majority of public EV charging stations are still free. The economic benefit of those stations is a no-brainer. Nonetheless, the convenience of charging overnight at home—even though your power meter is turning, followed by a utility bill at the end of the month—means that home charging will make up 80 to 90 percent of your charging. You want to be ready to roll with a full pack in the morning, right? Still, if you plan ahead, and use PlugShare  to do research, public charging can almost always be free.
To really know your cost in any circumstance, you need to know your car. For example, a 2011 Nissan LEAF can only draw 3.3 kilowatts, while a 2013 and newer version of the same model can draw 6.6 kilowatts (kW). A Tesla Model S can draw 10 kW, and with the dual on-board chargers option installed it can charge up to 20 kW at higher-powered charging stations. A Toyota Prius Plug-in Hybrid can only draw 2.3 kW. In general, most new plug-in hybrids draw 3.3 kW or less, while most new pure battery-electric vehicles (BEVs) draw 6.6 kW or higher. For that reason, a BEV gets a better deal from hourly pricing, but a plug-in hybrid gets a fair deal when paying by the kWh.
Among the public charging stations that require a fee, most of them use time (rather than kilowatt-hours or kWh) to set a price. For simplicity, let’s say that the price is $1 per hour. Most (but not all) of today’s EVs have a 6.6-kW charger. Logically, if losses are kept to a minimum, you will add 6.6 kilowatt-hours of energy to your battery in one hour. So, dividing that single dollar by 6.6, means you’re paying $0.15 for each kilowatt-hour. That’s pretty darn close to the national average for electricity—although maybe two or three times what you might pay if you’re on an EV time-of-use plan. Still, a full charge from empty to full on, say, a Nissan LEAF, will cost about a buck more in public—a reasonable amount to pay for the convenience.
Yet, there are potential gotchas. Some pure EVs, and nearly all plug-in hybrids, use a 3.3-kW charger—instead of the 6.6-kW equipment that has become standard. Charging by the hour with a 3.3-kW charger means you get only half the amount of juice for the same price. It’s still likely to be cheaper than using gasoline, but some of the advantage is erased.
More critically for all EV drivers, some charging locations leave the proverbial meter running until you unplug. So, even when the battery is full, and the electrons have stopped flowing, the price for that charging event continues to wrack up—sometimes creating a nasty bill. For multiple reasons, it’s best to unplug as soon as your battery is full.
So far, we’ve only been talking about Level 2 240-volt charging—for which you commonly pay either by the hour or it’s free. But for a 50-kW blast from a DC Fast Charger, it’s more common to pay for a “session.” That’s loosely defined as one time that you plug in. (See our guide to quick charging .)
For example, the AeroVironment network charges a fee of $7.50 for each DC Fast Charger session—or for the record, $4.00 for a session on a Level 2 charging station. The Blink network carries a similar fee of $5.00 for a Quick Charge (if you are a member) or $8 per session (for a non-member). The best economic case—besides the convenience of adding 50 or so miles in about 30 minutes—for using a Quick Charger by the session is when you can take your battery pack from empty to, say, 80 percent full.
Let’s assume that your battery pack is 24 kilowatt-hours. In this case, you could consume as much as 19 kWh for $7.50. In reality, it’s unlikely for your car to be completely empty, so the number of kWh will be less. In the optimal situation, you will be paying about $0.40 per kWh from AeroVironment, and $0.26 per kWh from Blink (as a member).
Your dollar-per-kilowatt-hour cost is probably double what you pay at home—essentially a pricey fee that you don't mind paying when you are in dire need of a charge, but not something that you would want to do on a daily basis.
There are a few other factors to consider:
In both of these cases, it’s quite difficult to make an apples-to-apples comparison of home electricity to public charging. Besides, you are paying something up front regardless of whether you ever take a charge—somewhat similar to annual fees at gyms, which earn a pretty penny from folks who have every intention of exercising, but rarely if ever sustain use of the facilities.
Here’s the takeaway: Use PlugShare to seek out free public charging whenever possible. It’s still widely available. If you really need a charge and you have to pay, don’t sweat the extra cost. Sure, you are likely paying double in public for the convenience of charging on the go—but it’s still way cheaper than gasoline. Public charging will rarely compete with home charging on an economic basis, but it’s part of the EV driving experience when you need that extra boost to get to your destination.