Remember this is based upon the amount of wiring actually IN the structure, so it would only work where it has some wiring in the walls, if you only have one jack or two it may only be working in the vicinity of that jack. You can use an EMF detector to see where it actually works, you can get a cheap one from home depot for 30$
The problem with most houses is poor wiring, one room may be a combination of two circuits, if you turn off that section of wiring using the circuit breaker you will see what I mean. I would use a small lamp, then flip the jumper off and watch which one turns off. Then place one Pest-offense on that circuit, do the same thing with each room or section of your house.. Older houses have bad wiring, so you may only have 1 to 2 circuits per section of the house, so only one device would be needed in that case. In newer homes they have several circuits around the house, generally 1 to 2 circuits per room. This is actually to keep your house from burning down by overloading the circuit breaker.
So newer houses need more Pest-Offense devices 3-4, while older houses may only need 1-2.
A circuit breaker inside the home should give you an idea how many wiring circuits exist, each jumper goes to a section of wiring, same with a building or a house. Generally when it says "Living room on a circuit breaker" it means that jumper handles the living room, which means you have to place one on that circuit to effect that area/space.
So, generally one per room usually works. This same idea has been used with network over power (By modulating the power frequency to act as a carrier signal), essentially they are just modulating the frequency of the wiring thus effecting the pests. It's kind of complex, but the theory behind this device is solid.
In my house I had to use 4 devices due to the modern wiring and circuit design. 1 for each section of the house. I confirmed that they were working by using an EMF meter and watching the pulse before and after. The signal changed when plugging in the Pest-Offense which tells me it does exactly what its supposed to do.
I plugged the devices in and in about a week all of the spiders/ants were gone. I also noticed the "sluggish" flies that another reviewer mentioned, I even tested this by turning the device off only to watch the fly move quicker, then turned the device on to watch it fumble about again. Kind of entertaining :) maybe it depends on the type of fly, it's just interesting to watch.