Based on a data-intensive weather-driven modelling approach, technically and economically optimal designs are derived for a simplified, highly renewable pan-European electricity system, which minimise the need for backup energy, backup capacity, transmission capacity and the levelised system cost of delivered electricity. The overall cost-optimal design, based on standard cost assumptions, relies on synchronised backup across the transmission grid and,comes with a renewable penetration of 50% with a rather high wind fraction of 94%. Given the current European electricity consumption, this corresponds to 600 GW rated wind power capacities, 60 GW installed solar power capacities, 320 GW conventional backup power capacity, and about five times today's installed transmission capacities. A sensitivity analysis reveals that the design and cost of the optimal system depend mostly on the assumed cost of wind capacity and fuel for backup energy. Lower costs for wind capacity, higher costs for backup energy and usage of otherwise curtailed excess electricity generation lead to a strong increase of the optimal renewable penetration. The sensitivity analysis is also used to find that a CO2 tax of over 100 sic/ton would be needed for the economic viability of carbon capture and sequestration. (C) 2015 Elsevier Ltd. All rights reserved.