Because air-conditioning systems typically are sized to keep a home cool on the hottest days of the year, they waste energy most of the time. Two-stage compressors, however, allow systems to run at either 100% capacity or 67% capacity, depending on the need. In this "How It Works" article, contributing editor Sean Groom explains how a two-stage compressor works. First, a scroll compressor pressurizes refrigerant and pumps it between indoor and outdoor coils. Next, in response to signals from the home's thermostat, a solenoid pilot valve either stays in the open position for partial-capacity operation, or in the closed position for full-capacity operation. Detailed illustrations show both the refrigerant-compression process and how the pilot valve determines operational capacity.