Although the Sense monitor only draws 4W, it must be installed on a 240V breaker for device detection purposes. Sense needs to monitor both phases of your electrical system and can only do so via a 240V breaker.
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.