What should future power grids look like? Our answer optimizes costs, stability and CO2. Renewables do not surpass 45-50%. By this point, over 70% of new wind and solar will fail to dispatch, while incentive prices will have trebled. Batteries help little. They raise power prices by a further 2-5x to accommodate just 3-15% more renewables. The lowest-cost, zero-carbon power grid, we find, comprises c25% renewables, c25% nuclear and c50% decarbonized gas, with an incentive price of 9c/kWh.