But when either replacing equipment, or specifying new installations, a variety of SEERs are available. For most applications, the minimum or near-minimum SEER units are most cost effective, but the longer the cooling seasons, the higher the electricity costs, and the longer the purchasers will own the systems, the more that incrementally higher SEER units are justified. Residential split-system AC units of SEER 20 or more are now available, but at substantial cost premiums over the standard SEER 13 units. The higher SEER units typically have larger coils and multiple compressors, with some also having variable refrigerant flow and variable supply air flow. The higher cost reflects the additional complexity