I think an actual resistor is not the correct answer what with V=RI, but there is a buck converter way to do it. You might even be able to use an MPPT controller--those things have a buck converter in them.
You said your own solar works with the Tesla, so can you make your Xantrex the input to that controller?
It might be an issue that the input voltage is rising instead of steady? Or maybe it would be steady at something above 25 in that set-up. Input has to be about 2v higher than the output ISTR.
Somebody here should be able to say if that would work.....
