as ulrich says, 2000w will eventually do the job....but 4000w would be a lot more efficient as far as your time goes. If you only have 120v available you could put 2 switch-controlled 2000w elements in your kettle and cut one off when you get to a boil. Otherwise, lots of people here (including me) use much larger PID/SSR controlled 240v elements with great success, but it is obviously a bit more work and money to put that together. If you have any thought of going to 10 gallon batches, you might as well start out big.