After my smoke detector 1 has after 15 years said goodbye with Dauerpiepesen, I opted for this product, which has cut off according to test one of the best. After 2 days the goods was there, Installation was simple, to the square Desgn you get used quickly. As an energy source, I use a lithium block, because their capacity is sufficient compared to ordinary alkaline block, approx 1-2 years, for about 5-6 years.