Considering that the charging system may bring the voltage of the system up to close to 15 Volts then yes it would be 37.5 watts. That's if it is dropping the whole load. Most likely it is less than that. But if there is a chance that if something breaks and it does drop the whole load you want it to be rated to do it. But that depends on how the circuit is designed.
Even so, a 40 watt 6 ohm resistor shouldn't be that hard to find and it shouldn't be that expensive. Try Radio Shack or look at some electronics supply places on the web.
Good luck.
Steve