From Halopedia, the Halo wiki
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. Some generators, like the PG000E Portable Electricity Generator, burn hydrogen. Nuclear fission and fusion power, as well as hydroelectric power plants and wind turbines, are also common forms of power generation. Both UNSC Army and Air Force units utilize air-transportable fusion power generators, like the MEP-PU-2550D/E Generator, to meet their mobile power requirements, particularly in harsh tactical environments where facility-integrated solar panels and wind turbines are insufficient or inappropriate. UNSC Firebases utilise large wind-powered generators to meet the needs of their operations.
The Covenant use generators of varying kind to power their structures, including High Charity. The most common power source is pinch fusion, a more advanced form of the fusion reactors employed by humanity. Smaller 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. 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 vehicles, such as their weapons, fuel, etc.