This unit emits a high frequency sound to repel rats and mice as the auditory stress distrupts their normal behaviour patterns. Can also be switched to repel crawling insects such as cockroaches, woodlice and earwigs (not ants). Will protect a single room up to 36 sq.m, or an open area up to 900 sqm. Remember ultrasound does not go through walls. Plugs into a mains socket and only uses about 3p electricity a week. Has no effect on cats, dogs, fish or birds. The pest clear has been particularly useful against rodents in thatched roofs.