I added a phase-fired controller to be able to control the heating intensity in the slicer settings. It's wired to the D9 output of the RAMPS board and controlled by the part cooling fan parameter.

Unfortunately it turned out like the one I bought works only in a range from 0-80% if controlled via pwm or 3-12V, but I think it should be still enough - I tested it and it still worked. 

Actually the metal melting system / hotend draws not very much power, just a few hundred watts depending on the resistance of the working circuit.