A baby monitor's job is to transmit recognizable sound and, in the case of video models, images. The challenge is to find a monitor that works with minimal interference—static, buzzing, or irritating noise—from other nearby electronic products and transmitters, including older cordless phones that might use the same frequency bands as your monitor.
For the parent devices of dedicated monitors, the battery life ranged anywhere between 6.75 and 12.75 hours. The Wi-Fi options are harder to gauge given that the battery life depends on the kind of device used, whether or not it is being used for other applications simultaneously, and how old the battery is in the device. In general, however, we feel it is relatively safe to say that most will work longer than the best dedicated monitor battery if the device is dedicated for use with the monitor only and is not running other applications simultaneously.