In this article, contributing editor Sean Groom explains how to choose the most efficient central air-conditioning system for your home and your budget. In order to do so, it's necessary to understand four measurements: cooling capacity, which is described in tons; British thermal units (Btu); the energy-efficiency ratio (EER); and the seasonal energy-efficiency ratio (SEER). Most residential central air-conditioning systems are split systems, which means that they have an indoor and an outdoor unit. For a system to achieve its advertised SEER rating, the two units must be matched properly. A system with a two-stage or variable-speed compressor and with a SEER rating between 19 and 24.5 is the most efficient, and a variable-speed fan on the air handler contributes about 1 SEER to the system's performance. Throughout the article is a series of shopping tips on topics such as how to understand the advertised SEER rating of a model line from a single manufacturer, why higher efficiency means fewer sizing choices, and how to understand manufacturers' claims about refrigerants. In a sidebar, Groom looks at SEER ratings in three categories--economy, efficient, and very efficient--and describes the changes manufacturers make to systems in order to increase their efficiency. In another sidebar, Groom explains how to find a good installer and why that is important.