In business school, professors like to proclaim that “there is no such thing as a free lunch.” I hated that saying (and still do), because Boston University gave us a bunch of free lunches when we had seminars or when they threw us happy hours. I also got a ton of free lunches when I worked for a mortgage company and everyone was buying all those houses back in 2002 and 2003; our parent company was making so much money we had catered lunches in our kitchens every day. It was magical.

So let’s be clear…there are such things as free lunches; they just don’t always last.

Such is the case with charging your electric vehicle in public places. The Wall Street Journal reported this week about plans from companies providing now-free charging stations to start making people pay for charging. But because the industry is still emerging and the infrastructure economics of electric vehicles is still evolving, providers aren’t sure how much to charge. They do know that “free” isn’t the most sustainable option.

ECOtotality, a large installer of public charging stations, will offer a three-tiered payment structure beginning in the fall. Non-registered users will pay $2 an hour, drivers who register with the Blink network will pay $1.50 an hour, and Blink Plus members will pay $1 an hour after ponying up their $30 annual membership fee.

NRG Energy has built 28 fast-charging stations in Texas and is using a subscription model for its energy. $39 a month gets a driver unlimited access to their public chargers that can charge cars in half an hour or less, and $89 a month for unlimited use of the stations.

Electric vehicle industry experts have agreed anything more than $2 an hour to charge an electric car eliminates the savings from not using gas.

So go on and get your free car electricity while it’s available on the super cheap; it’s just one of those economic sweeteners that goes away when companies sink their claws into you.

Maybe that free-lunch saying should be amended:

“There is such a thing as a free lunch, but it likely won’t last. So take advantage of it, don’t question it, and if there are leftovers in your company’s kitchen, bring them home for dinner.”