This paper introduces a novel approach to estimate the maximum short-term output variability that an arbitrary fleet of PV systems places on any considered power grid. The paper begins with a model that demonstrates that the maximum possible variability for N identical, uncorrelated PV systems equals the total installed capacity divided by root 2N. The paper then describes a general methodology that is applicable to arbitrary PV fleets. A key input to this generalized approach is the correlation, or absence thereof, existing between individual installations in the fleet at the considered variability time scale. In this respect, the article includes a presentation of new experimental evidence from hourly satellite-derived irradiances relating distance and fluctuation time scales in three geographic regions in the United States (Southwest, Southern Great Plains, and Hawaii) and from recent high density network measurements that both confirm and extend conclusions from previous studies, namely: (1) correlation coefficients decrease predictably with increasing distance, (2) correlation coefficients decrease at a similar rate when evaluated versus distance divided by the considered variability time scale, and (3) the accuracy of results is improved by including an implied cloud speed term. (c) 2011 Elsevier Ltd. All rights reserved.