We model many combinations of renewable electricity sources (inland wind, offshore wind, and photovoltaics) with electrochemical storage (batteries and fuel cells), incorporated into a large grid system (72 GW). Our model evaluated over 28 billion combinations of renewables and storage, each tested over 35,040 h (four years) of load and weather data. We find that the least cost solutions yield seemingly-excessive generation capacitydat times, almost three times the electricity needed to meet electrical load. This is because diverse renew- able generation and the excess capacity together meet electric load with less storage, lowering total system cost. At 2030 technology costs and with excess electricity displacing natural gas, we find that the electric system can be powered 99.9% of hours entirely on renewable electricity, at costs comparable to today's but only if we optimize the mix of generation and storage technologies. This excess installed capacity of volatiles lead to vast amounts of curtailed energy.