A compressor (1) including a compressor mechanism; an input line (12) for providing gas to the compressor; and an output line (14) for providing compressed gas from the compressor. The compressor may supply gas at a first pressure or at a second pressure, by variation of the charge pressure within a gas circuit. A buffer volume and arrangement of valves, contained within the compressor, facilitate the change in static charge pressure. The electrical power drawn by the compressor is reduced when the charge pressure is reduced. Changing the charge pressure in the compressor also varies the cooling power delivered by the refrigerator. Therefore, this variable charge compressor can be used to reduce the electrical power drawn by an MRI system when it is in standby, and the full refrigeration capability is not required. Also, this has the effect of reducing wear and increasing the life of certain components within the refrigerator and compressor.