Six Nissan Leaf EVs power an office in Japan.

At least now we know the answer to that age-old joke, "How many Nissan Leafs does it take to power an office building?" The answer is a tidy half-dozen.

The Japanese automaker has been testing its "Vehicle-To-Building" system, in which the building pulls juice from the cars during the peak-use hours when power is most expensive, and then shoots back some power when the grid prices drop. Nissan says the system is set up to ensure the cars are fully charged by the end of the workday. If you leave work early? That may be a different situation.

The company started testing the system at its Nissan Advanced Technology Center in Atsugi City, Japan, in July, and found that the cars allow the building to cut peak-hour electricity use by 2.5 percent. Annualized, that means savings of about a half-million yen (about $4,800 US) in electricity costs.

Nissan is also working on a "Leaf-to-Home" power-backup system, in which the car can be used as a residential-power backup source in order to reduce peak hour use and provide power in a blackout. That system was first unveiled in the summer of 2011 at a house built in front of Nissan's global headquarters.