To demonstrate technical viability of photovoltaic modules in central, grid connected energy systems, ARCO Solar, Inc. has designed, installed and is operating two photovoltaic power plants on the megawatt scale. These systems use two-axis tracking. The first generation plant in Lugo (Hesperia), California, with a nominal rating of one MWpk (DC)" was installed in 1982 in the Southern California Edison Company grid. The second system, rated at 6.4 MWDk (DC), is located in the Carrisa Plain in California and connected to the Pacific Gas and Electric Company grid. Based on the cost and performance data from these installations, an assessment of the current status and future needs of large scale photovoltaic energy systems is made. With each new system, improved techniques of design, installation and system integration have been developed. Expectations have been confirmed as to the performance and adaptability of solar cells, especially the ease of incremental increases in capacity when needed. Modular photovoltaic systems have been found to be easy to build and operate, and to be highly reliable. Prologue: Technological advancement usually requires good science and logical engineering. In the main, faith, persistence and feel are also required. Rule: The balance-of-system costs for photovoltaic energy systems equal photovoltaic module costs. Photovoltaic systems have progressed to their current stage of high promise because of faith, persistence, feel and belief in this rule.