This is the simplest and cheapest device to easily enable connection of a generator to your house (short of just disconnecting the wires and attaching them directly to your generator--not really recommended in the panic of the moment in a power outage). It is used on one circuit only--a 15 amp one. The obvious circuit to use it on is for your furnace (forced air or, like me, hot-water radiators). If you have a long power outage in the winter, you can run your furnace from time to time to prevent the house from freezing.
While you can of course plug in a conventional gasoline-powered generator, I use this transfer switch to plug in a 12 volt power inverter during power outages. The inverter generates 120 volts from my car battery. If you get a big enough one (maybe 1000 watts) it will sustain the brief initial "transient" surge of power needed to get an electric motor going. The actual power needed after than initial surge is pretty low: about 300 watts. It is probably best to run the car's engine while doing this, so as to be sure to have full power and to prevent the car battery from running down. This method worked on my old furnace with a cheap power inverter. On my new furnace (a fancy one with electronic controls) there was a twist: I found I had to use a "Pure Sine Wave" inverter, not a standard (so called "modified sine wave", which is closer to a square wave) one, to make it start. (a refrigerator and sump pump don't need pure sine wave).
With a power inverter one can avoid the hassle, expense, smell, gasoline-storage hazard, and pollution of a gasoline generator that one almost never needs to use.
The instructions for this transfer switch assume you will be connecting this switch to your main fuse box. But I found that it's more convenient to connect it on the line leading to your furnace--a method shown in illustration on the box but not detailed in the instructions. Use great care to be sure to wire it up correctly.