|There is more information available on this subject at Generator on the English Wikipedia.|
Generators are devices that use a series of physical and chemical changes to convert energies such as mechanical energy into usable form, most commonly electricity.
Though little is known about their operating principles, the Forerunners used generators to power their installations. For example, the Phase Pulse Generators were used to power the firing system of the Halo installations.
Humans use generators of varying kind to generate electricity, though not much is known about how generators in the 26th century operate. Human-made generators are also a Forge item in Halo 3. They can be placed in certain maps, such as The Pit, and Orbital.
Covenant generators expand upon a single Plasma Battery to multiply its yield, which is transferred and stored in Covenant barrels that are attached to the base. These barrels have seen unattached to the generating bases which are probably transported and used to fuel Covenant vehicles.
The armor plating on these devices prevent tampering or destruction by the player on Campaign mode. It is likely that the fuel that the device generates into the detachable and replaceable tops might be plasma-based. This means that they could be used to fuel the systems on Covenant, such as their weapons, fuel, etc. In Halo 2, these generators can be found on Burial Mounds and on Metropolis, near the Marine command post.