It's all a matter of waiting a bit for 'ramp up'. I have two 5500 watt elements for my 20 gallon system. I ramp up with 11,000 watts but a roiling boil is maintained easily by about half power of one element.
And, if you begin heating as soon as your element is covered, while sparging, you'll probably not have to wait that much at all.
BTW, after years with LP, I'd recommend you move entirely to electric!