The Western Wind and Solar Integration Study is a multi-year study of the electrical implications of integrating significant volumes of wind and solar power on the US portion of the Western Interconnection. The study overview, together with detailed and summary findings, is available on the U.S. Department of Energy's NREL site.
The quotation, in the graphic above, is taken from Phase 1 of the study which was prepared by GE Energy: The quote in full;
"The first phase of the Western Wind and Solar Integration Study investigated the benefits and challenges of integrating up to 35% wind and solar energy in the WestConnect subregion and, more broadly, the Western Interconnection. The study showed it is operationally possible to accommodate 30% wind and 5% solar energy if utilities substantially increase their coordination of operations over wider geographic areas and schedule their generation and interchanges on an intra-hour basis."