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.
Being expensive and taking up a build slot, commanders who wish to use the Shield Generator must choose between a strong defense and other options. It is powerful enough to withstand a direct hit from the UNSC Spirit of Fire's Magnetic Accelerator Cannon. It is advised to build one early on in the game and/or after you have established a strong army.
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.
However, there is a glitch where after a Shield generator is made, all structures in the base that have already been made lose life points.