It turns out that, in a pinch, a Honda EU6500iS generator can charge a Tesla Model S.

"It's typically very hard to get the Model S to charge from a portable generator (they don't produce clean enough power) so this was mostly to prove you can do it with this particular model if you absolutely needed to."

Says the uploader of this video.

Honda lists the EU6500iS with an MSRP of $4,499.95

Some specs on the unit are below.  Additionally, you can follow this link to Honda's site for more details on the generator used in this video to charger the Model S.

EU6500iS Specs

EU6500iS Specs