I looked into getting the top-recommended first alert brand (which links to amazon, not the manufacturer) and there seems to be a consistent problem with customers being sent the wrong version, or sent empty or faulty products. Returns are not accepted. When customers follow up with first alert, they will do nothing to stand behind their product. The rest of the info in this review was great and informative, but I felt the need to point out the flaws in your 1st choice results before someone invests in a product that won’t stand up in an emergency when they need it most.