This already exists, although not for Teslas. It's called V2G (vehicle to grid) and it's already happening in the UK with the Nissan Leaf.

Not only is this a backup for a rare outage but it makes you money by charging when power is cheap and selling back when expensive. Occasionally the price of power goes negative and you are paid to charge.

https://electricinsights.co.uk/ is good for exploring the energy data and pricing.