What is the optimal mix of wind and solar in a low-cost, zero-carbon power grid? We find renewables cannot surpass 45-50% due to curtailment, which trebles prices. Batteries help little, under grid conditions. Decarbonized gas is the best backstop. A grid of 50% decarbonized gas, 25% renewables and 25% nuclear has the lowest incentive price, at 9c/kWh.