To integrate solar energy into the electricity grid reliably and efficiently, different forecasting techniques are used to produce forecasts at various forecast horizons using data of different levels of aggregation. For example, power generated by distributed photovoltaics (PV) can be disaggregated in a geographical hierarchy into transmission zones, distribution nodes, PV plants, subsystems and inverters; forecasts are required at all of these levels to facilitate different power system operations and power plant management. Due to the different information sets visible to different players in the hierarchy, such as PV plant owners and independent system operators, the forecasts produced at different aggregation levels are often not optimal. Furthermore, these forecasts are aggregate inconsistent, i.e., forecasts made using data collected at lower levels do not add up exactly to the forecasts made using higher level data. In this paper, the state-of-the-art forecast reconciliation techniques are explored. By minimizing the trace of forecast error covariance matrix, the base forecasts obtained across the hierarchy can be optimally reconciled. The reconciled forecasts not only aggregate consistently, and thus lead to collaborative decision making, but also improve the base forecasts at each level significantly. The merit of the reconciliation framework goes to the fact that it does not require any additional information other than those base forecasts. Furthermore, forecast improvements are independent of the base forecasting methods. In other words, once the base forecasts are generated with our favorite methods, the reconciliation will most likely improve those forecasts further. The reconciliation techniques can be applied to a variety of hierarchies with different forecast horizons. The empirical part of this work considers two examples, namely, an NWP-based day-ahead forecast reconciliation over 318 simulated PV plants in the state of California, and a spatio-temporal statistical 1 min-ahead forecast reconciliation over 17 irradiance sensors on the Oahu Island, Hawaii. It is shown that reconciliation benefits both the gird operators and PV system owners by bringing more accurate forecasts, and thus motivates information sharing in an electricity grid. (C) 2017 Elsevier Ltd. All rights reserved.