In this paper, we propose Access Point Channel Capacity (ACC) as a Key Performance Indicator (KPI) for Radio Resource Management (RRM) algorithms in 802.11 Wireless Local Area Networks (WLANs). Introducing such a KPI is important especially for performance evaluation of 802.11 WLAN Self Organizing Networks (SON). ACC is a low complexity KPI that provides accurate per-AP estimate of the MAC layer potential throughput in the WLAN. The advantage of the proposed metric is that it can be derived directly from network statistics collected periodically from the access points. ACC is a vendor agnostic KPI since the required statistics are widely supported by all AP vendors and also TR-069 data model.