Historically most of the gas turbines that have been used for power generation have been relatively large, with electrical outputs of over 1 MW. Many have outputs of several hundred megawatts. However there is a group of much smaller turbines, called micro- turbines. These turbines are intended for use in distributed genera- tion applications where they supply electrical energy, and often heating or cooling too, to group of local energy users. For the smallest of these microturbines, the users might be in a single house- hold. Even smaller units, called ultra-microturbines are being devel- oped for portable use, as a replacement for batteries.
There is no standard definition of a microturbine. Some commenta- tors include all turbines with an electrical output of less than 1 MW. This covers a broad field since the smallest of them can be of the order of tens of millimeters in diameter and have outputs of 10 to 100 W. For stationary applications the minimum size is likely to be 1 to 5 KW, sufficient to provide power for a small domestic dwelling. However, most of the commercial microturbines available are much larger with sizes ranging from 30 to 500 kW. These larger machines can also be deployed in parallel to provide even larger microturbine installations. The market for these devices is still evolving and the may take several more years to establish a firm position. Many commercial microturbine systems are still under development.