That's a really good idea. You typically only need it on long trips, and then it can also be a normal trailer to load all your gear into. Meanwhile when you get to your trip destination, now you have a generator to run electrical devices without having to worry about running down the car's battery.

And it's normally at home, so if the power goes off for a week you can still run your house and charge your car.

You should just be able to rent the trailer if you need it a few times a year. Parking space for a trailer that you're never using is going to cost you too much in big cities.

This is brilliant.

You would need a 20-30kw generator+fast DC charger as the built it AC charger in most electric cars is not fast enough to maintain a charge at highway speeds (especially while towing something).

Like other commentators have mentioned, if a u-haul like network of these were made, if you ran out of charge somewhere, they could drop one of these off to get you going again, as a depleted battery pack will charge slowly, making the van full of batteries potentially more expensive (you would have to pay for an extra 15-30m of their time).

20-30kW might be a bit higher than needed unless the trailer is oversized to provide extra cargo capacity in addition to carrying the generator. You don't need a generator big enough to prevent the battery from draining at all; you just need to extend the range enough to cover a full day of driving while offsetting the extra drag from the trailer. 10-15kW is probably plenty for a trailer that only carries the generator: that would double the highway range of a Model S and would weigh about 700 lbs while being compact enough to have very little aerodynamic drag. A 30kW generator would be enough to double the range even if the trailer doubled the energy consumed per mile, and that's only going to happen with a pretty large trailer.

The Model S is fairy efficient and aerodynamic compared to other EVs.

If anyone did this, they probably would offer 15kW and 30kW versions, so both passenger cars and light trucks would be covered.

Thats actually a pretty sweet idea. Only take the generator with you when you need it.

Make them pluggable into the car (vs. towable) and standardize on the interface, and there could be market for renting the generators (like e.g. you do with city bikes - take them from one station, drop off on another).

Attach it to the tow hitch framework like a bike rack. :)

I suspect that would really mess with the load dynamics on most passenger type cars, as the generator would be pretty heavy. I don't think you'd be able to lift a big enough one into the trunk area, and attaching it to the back of the car would be a pain too. I think towing solves a lot of issues for the idea.