I wasn't sure on brewing with induction so I went to the local K Mart and bought their
$50 cheapie, I've been pleasantly surprised at how well it's worked. I've spilt wort on it a few times and dropped it a couple of times and it hasn't died yet. Honestly if it did I think I'd just buy another one. The instructions are in
Engrish but once you work out how it goes it's really easy to use.
Be aware that the rated power is the input power to the RF circuit, these are never 100% efficient so effective output power is a bit less (90% is as good as it gets). The 2 kW unit handled 15 litres quite well though it was a little slow to come to boil. I anticipate it will struggle to cope with 25 litres so I am in the process of putting a second (resistive) heater in the mash / wort kettle. What I think will work is to leave the induction heater on an appropriate setting and control the resistive heater with an STC1000 to achieve the required mash profile. I bought one of Mattrox's profile controllers for this purpose. Similarly, I expect to have to use the resistive heater and the induction heater to get to boil in a reasonable time then cut back to just the induction heater to maintain the boil.
Based on this I think a 3 kW unit will really struggle with 50 litres.