So far, in my keggle, I have 2 5500w 240Vac. I turn them on full power and can boil 15 gallons from 70deg in about 35 minutes. I have it wired so that I can turn either element on, off, or have them wired in series. So possible powers are 11000w, 5500w, or 2750w.
I've found that I can keep a fairly good boil at 15 gallons with just 2750, but if need be I can bump it up to 5500, but I've found that doing that tends to make big enough bubles to splash out of the top of my keg.