Can I use the On/Off Plug-In Switch outside?

The On/Off Plug-In Switch is not designed for outdoor use.  However, with some careful planning, you might be able to use it outside safely.  There are a few things to keep in mind when using Z-Wave devices and electronics in general outside, each of which we will discuss closely:

1. Extreme Heat and Cold
2. Humidity/rain
3. Z-Wave network coverage

Extreme Heat and Cold
In extremely cold conditions, three things happen in electronics:

1. Water from the atmosphere will condense on the surface and either short the circuit or freeze and expand, causing components to come loose and malfunction.
2. Since different materials expand and contract with changing temperatures at different rates, soldered and otherwise attached components can disconnect from each other.
3. Batteries start to discharge and become ineffective.

Batteries are not a concern with the On/Off Plug, but condensation and thermal expansion are.  Since the On/Off Plug wasn't designed to withstand these conditions, the cold could damage it, though it has better chances of survival in drier climates.  

The heat should not interfere with proper functioning.

Humidity/Rain
Direct rainfall will be an issue with the On/Off Plug, and high humidity could cause problems in colder climates.  The On/Off Plug should be kept out of direct rain at all times, and excess humidity should be avoided if at all possible.

Z-Wave Network Coverage
Z-Wave signals become weaker as they travel over distances (called "attenuation,") and this is especially true when passing Z-Wave signals from the inside to the outside.  The thickness and construction materials (iron bars, concrete, etc.) seen in exterior walls make it less friendly to Z-Wave signals than typical interior walls, and the potentially higher humidity levels outside will also interfere with your Z-Wave signal.  It is best to include multiple extenders or plugged in devices near the external wall closest to your installation location, but still inside, so a stable connection is formed between the main controller and your device.