  The transition to a future electricity system based primarily on wind and
solar PV is examined for all regions in the contiguous US. We present optimized
pathways for the build-up of wind and solar power for least backup energy needs
as well as for least cost obtained with a simplified, lightweight model based
on long-term high resolution weather-determined generation data. In the absence
of storage, the pathway which achieves the best match of generation and load,
thus resulting in the least backup energy requirements, generally favors a
combination of both technologies, with a wind/solar PV energy mix of about
80/20 in a fully renewable scenario. The least cost development is seen to
start with 100% of the technology with the lowest average generation costs
first, but with increasing renewable installations, economically unfavorable
excess generation pushes it toward the minimal backup pathway. Surplus
generation and the entailed costs can be reduced significantly by combining
wind and solar power, and/or absorbing excess generation, for example with
storage or transmission, or by coupling the electricity system to other energy
sectors.
