I gave up on electronic killers, they only get one or two rats. I made a fairly large wire cage, with mesh too small for rats to escape, with a drop down shutter. Inside I put a thin board, suspended on a hinge one end, and the other supported by a spring. Put grain in it, when enough rats are in it their weight overcomes the spring and the floor drops a few mm. This closes a microswitch and a small accumulator triggers a solenoid and the gate drops. You are left with a cage with several (the most I caught in one go was ten) rats in it. You can then dispose of them. Safe, has a simple electronics element, and won't kill things that you don't want or expect to.
A large piece of single sided PCB etched into a form of well spaced tacks, alternate ones to ground and a HV supply works, too, put in a cage to keep out cats and other pets, but if only one rat enters, it gets zapped, and being fairly intelligent the others, err, smell a rat (sorry....) and keep clear. It also zaps birds and other "nice" critters, so is not very selective. For lots of rats the cage traps, rat bait, fenn traps or a good small terrier are alternatives.
I am unsure if an electric fence energiser has enough clout to kill a rat, probably not instantly. Possibly worthy of investigation.
I have a 6kV 1.5 amp DC supply and a vacuum relay, but I think that's a bit OTT.....