The shield technology used by the Shield generator is repurposed Forerunner technology and requires vast amounts of power to use. Once the shield is drained, the buildings underneath the shield begin to take structural damage. Over time, the shield will begin to regenerate back up to full strength. If multiple shield generators are built onto one Citadel, the shields become much more durable. It is a good idea to place the shield generator to the rear of your base, in order to protect it from enemy fire once the shields are down. The shield generator has no UNSC counterpart.