The Missile Battery is a buildable Base Facility and provides defense for your bases when under enemy attack.
Upgrades to Laser Battery. Auto-Upgrades when relevant tech is researched.
Xenopedia[]
The Defence Battery is a point-defence structure that provides your base with protection against UFOs. In the event that your base is attacked by enemy forces, your defence batteries will each take a shot at the attacking craft with a 50% hit chance before the ground assault begins.
Several defence batteries may be able to destroy an attacking UFO before it lands, averting the attack completely - but inflicting any damage on the UFO at all will reduce the size of the alien ground forces appropriately (to a minimum of 50%). These batteries upgrade automatically as you research more advanced weapon technology, so will not become obsolete over time.