1500watts / 120 v is a good choice or close to it. Which one would depend on size of RIMS tube and the size of the element. Check out http://www.plumbingsupply.com/elements.html
A 240 element run at 120v will give you 25% of the rated power. It works, it is just that for my system it would be a bit underpowered. That's why I mentioned in my earlier post about knowing the size of the RIMS tube.