Cactus energy (サボテンエネルギー Saboten'enerugii, also Saboten energy if not fully translated) is a form of electrical energy produced from cacti in the Seihou Project that surpasses nuclear power. Its goal is similar to that of nuclear power, where many things are powered by cactus energy, such as VIVIT herself and Gates' aircraft. VIVIT thinks that the name for this energy is "stupid".
Many decades ago on Earth, for some reason the environment was getting worse, and all its vegetation got extinct, except for the cactus. The Cactus Company, founded by Erich, then tried to extract energy from cacti for humanity to continue, but over time they were unable to stable the energy, causing a disastrous explosion. The uncontrolled energy turned Erich into a cyborg and got Vivit (Erich's daughter) trapped in what is called the "Other Dimension". Decades through, the company managed to successfully control the energy, so they once again prospered. During the time, the company created a maid-type robot called VIVIT, who was based on Vivit.
In reality, it's theoretically possible to produce energy from cacti, but not in the same way as Seihou Project's cacti. Some species of cacti such as Sclerocactus glaucus can be a source of hydrocarbons that can be converted into fuel sources similar to ethanol or petrol. However, processing those hydrocarbons creates environment-unfriendly byproducts.