Can be driven directly using mechanical energy (water, car/truck motor) or with electrical energy. Efficient up to 60% of Carnot's theoretical limit (as evaluated in ASHRAE testing conditions: evaporation temperature of -23.3 °C, condensing temperature of 54.4 °C, and ambient temperature of 32 °C) based on some of the best commercially available compressors, as produced by manufacturers Danfoss, Matsushita, Copeland, Embraco, Bristol and Tecumseh. However, many refrigeration systems use compressors that have lower efficiencies of between 40-55%, since the 60% efficient ones cost almost twice as much as the lower efficiency ones.