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.