The main distinction between the two is the way they create fog. Electric ones are usually (but not always) ultra-low volume or cold foggers. They produce high pressure inside a special chamber to turn the liquid insecticide into tiny particles and spray it into the air. This solution works best with water-based insecticides. The gas-powered ones are known as thermal foggers: with heat, they vaporize the insecticide inside the heating chamber and release it. However, there are such gadgets that use the thermal technique but are electric, and there are even fuel-powered machines with a motor.