Whether for your home or car, plug-in air fresheners basically all work the same: Pick your favorite scent, then simply insert the fragrance refill into the warmer unit and plug it into an outlet. Plug-in air fresheners typically last for weeks before needing to be refilled, and some also allow you to adjust the intensity of the fragrance.
In fact, plug-in air fresheners are pretty much designed to be left for longer periods of time. They consist of a small unit with a moulded plug. A typical plug-in air freshener uses 4 watts. Not very much, but left on permanently, this will use 35 units of electricity per year.
The warmer units do not get hot enough to cause a fire or burn your skin to the touch, which makes them attractive to use without fear of burning down your space. But, you shouldn't leave these air fresheners plugged in forever, either. Always make sure they are plugged in up right so the oil does not leak out of the unit as most oils can stain floors and fabrics. It is an easy way to scent your home and make it an easy stress free and relaxing space.