SP-15A 220 single phase induction forge heating 1.5" bar stock.

This is Grant Sarver's Model 15 that so many people have. Itis a reliable machine used by hundreds of satisfied customers, Grant rated this at 1.5" bar stock. It will heat thicker stock, but it takes a little longer. As you can see in the video, heating is a surface phenomena and time must be allowed for it to soak through the work. One needs to either keep the heating point moving or lower the field intensity to keep from melting and oxidizing the surface before the center is hot.
His bigger machine, the SP-25 he rated at 2.5" I have a couple videos of both the machines heating a piece of 1.75" bar stock so you can compare the power output.
See inductionforges.com for more information.