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Aggregate ramp rates of 86 distributed photovoltaic (PV) systems installed in Southern California were analyzed and compared to irradiation measured at five ground stations and estimated from satellite. Irradiation data was converted to power output using a PV performance model to evaluate whether widespread on-line metering and telemetry of PV systems is necessary to track output of distributed generation for resource-adequacy applications. The satellite data were able to closely follow the aggregate power output and detect the timing of the ramps while the five weather stations were not as accurate due to smaller number and non-representative geographical distribution with respect to the PV sites. Over one year, the largest hourly aggregate ramp was a 50% increase based on the Performance Test Conditions (PTC) rating but ramps over 30% of PTC occurred only about once per day. The effects of specific meteorological conditions, such as coastal marine layer clouds and frontal system effects, on occurrence of large ramps were investigated over the area using satellite imagery. Evaporation of morning marine layer clouds caused a disproportionally large amount of up-ramps.