In this paper, we investigate the spatial dependence of variations in power output of small residential solar photovoltaic (PV) systems in a densely populated urban area (≈100km 2 ) in and around Utrecht, the Netherlands. Research into the geo‐statistical behavior of this kind of randomly spaced collection of PV systems is complementary to other studies in the field of compact regularly spaced MW‐scale PV plants. Fluctuations in power output between PV systems are correlated up to a certain decorrelation length. Decorrelation is reached (within 1 − e −3 ≈95 % ) in an exponential model and the spatial scale ranges from 100 m to approx. 15 km, with a mean value 0.34(±0.2), 2.6(±0.3), and 5.0(±0.5) km for measurement time step of the time series of respectively 1, 5, and 15 min. These length scales are typical for an urban environment and is important for reduction of variability in aggregated output variability of PV systems. Furthermore, the distance‐independent variability still itself was found to be strictly linearly dependent on daily mean variability values. This is a good validation of the decorrelation of inter‐system ramp rate correlation over distances longer than the characteristic decorrelation length. Copyright © 2014 John Wiley & Sons, Ltd. Distance scales over which inter‐system ramp rate variability becomes distance independent are investigated in realistically distributed urban rooftop photovoltaic systems in the Netherlands. It was found that this distance scale varies between 100 m and 15 km, depending on the overall variability of the solar irradiation. This knowledge is relevant for in situ irradiation and power output forecasting of solar photovoltaics.