Variable Speed Air Compressors use electronics to vary the the electrical frequency the air compressor's motor sees. By doing this the speed of the electrical motor can be sped up or slowed down depending on the amount of air required. This process saves energy reducing the operating cost of the compressor in most cases. Variable Speed Air Compressors should be used when air requirements fluctuate substantially from hour to hour and day to day. Most Energy Utilities offer some sort of rebate that can be applied to the purchase of a Variable Speed Air Compressor.