We develop a metric to quantify the sub-hourly variability cost of individual wind plants and show its use in valuing reductions in wind power variability. Our method partitions wind energy into hourly and sub-hourly components and uses corresponding market prices to determine variability costs. We use publically available 15-min ERCOT data, although the method developed can be applied to higher time resolution data if available. We do not estimate uncertainty costs though our metric can separate integration costs into variability and uncertainty components. The mean variability costs arising from 15-min to 1-h variations (termed load following) for 20 ERCOT wind plants was $8.73 +/- $1.26 per MWh in 2008 and $3.90 +/- $0.52 per MWh in 2009. Load following variability costs decrease as capacity factors increase, indicating wind plants sited in locations with good wind resources cost a system less to integrate. Twenty interconnected wind plants had a variability cost of $4.35 per MWh in 2008. The marginal benefit of interconnecting another wind plant diminishes rapidly: it is less than $3.43 per MWh for systems with 2 wind plants already interconnected, less than $0.7 per MWh for 4-7 wind plants, and less than $0.2 per MWh for 8 or more wind plants. (C) 2012 Elsevier Ltd. All rights reserved.