Although the Sense monitor only draws 4W, it must be installed on a 240V breaker for device detection purposes. Sense needs to monitor the voltage across both legs of your electrical system. Installing on a 240V breaker ensures that this is done correctly.
If there is not enough space in your electrical panel for a new 240V breaker, there are several possible approaches depending on your setup. Please consult a local electrician for guidance, as local electrical codes may disallow certain solutions, such as piggybacking on an existing 240V breaker.