V2L is able to provide around 3.5kW of a 13A plug, which isn’t enough to power a whole house. To do this in any meaningful wa you would need “Vehicle to Home” capable car and a bi-directional charger, which are not widely available yet.
Now thats out of the way lets look at what you can achieve, which is not alot but, say you wanted to power a few lights, fridge, TV and a microwave say.
1- An automatic changeover switch, instananeously disconnect from the car once the power comes back on.
The issue with this is (a) the potential of 13A supply the whole house, requiring you to use judgement on what appliances can stay on (b) A 13A plug hanging outside. (c) the plug not plugged in at the time the changeover happened.
2- The other way would require a small consumer unit wired to an circuit of backup lights and a a double socket. In the event of a power cut you plug manually into the car to power this circuit –
or an extension lead to power a couple of lamps and appliances. (most appropiate)