Even better, a very similar system is currently being implemented by Tesla (under the name Tesla Virtual Power Plant). It's for their Powerwall product rather t

Even better, a very similar system is currently being implemented by Tesla (under the name Tesla Virtual Power Plant). It's for their Powerwall product rather than the cars, but I think it's safe to assume they'd design a similar system for their cars. It looks like the Virtual Power Plant program lets you configure a backup reserve, so you can choose how much of the capacity you keep for yourself and only push a small portion of the battery's capacity to the grid.