Clouds are the dominant source of PV power output variability and their velocity is a principal input to most short-term forecast models. A new method for deriving cloud speed from data collected at a triplet of sensors at arbitrary positions is presented; cloud speed and the angle of the cloud front are determined from the time delays in two cloud front arrivals at the sensors. Five reference cells at the 48 MW PV plant at Henderson (NV), were used to provide two different triplets of sensors. Over a year of operation cloud speeds from 3 to 35 m s(-1) were obtained. Cloud speeds are validated using cross-correlation of power output from 96 inverters at the plant. Overall bias errors were less than 1% and the overall annual RMSE was 20.9%, but results varied with season. (C) 2013 Elsevier Ltd. All rights reserved.