If you can manage it, a 2000w element is likely a start for smallish boils, you will need a 20 amp circuit. Problem with being watt challenged is that it will take near forever to boil, and then be a weak boil at best. Of course, a big 240v, 5500w element on a controller is preferred, but a simple 2000w element will do some work. 2 elements at 2000w each is even better. 1500w will be slow and require an insulated kettle w/ marginal results.
I also have an electric turkey fryer that is only 1650 watts. It will boil 6 gallons but it takes a looong time and needs to be partially covered to maintain a boil. I guess what I'm saying is that being on the low end of wattage is not fun and adds time to your session.
To your question, 5000w will boil 10 gallons.