I run the Chinese 1800 Watt ZVS IH at 36 VDC input and everything stays cool over a course of 10 minutes. I would say it can run stable at this power input. The 5 minute test at 40 VDC showed resonant capacitors heating up.
Be sure check out the other videos and blog posts about this 1800 Watt ZVS induction heater. Firstly you could start by watching the Unboxing a Chinese 1800 Watt Induction Heater. Secondly watch the 1800 Watt Induction Heater, 5 Minute Stress Test that pushed it to the limits. Thirdly check out how I finished the project 1800 Watt Induction Heater, Boxed Up With Server PSU + Test.