I'm sure you didn't mean this, but the first time I read your email, I got the impression that you were going to connect an inverter to your RS485 output to actually power the lights !!

I seem to recall that a company called Black Box used to sell switching units that already contained the relays etc. You need a computer mag' from the early 80's, something like an Acorn Atom or Nascom used to do this sort of thing...

Regards
Simon