Negative pricing from solar occurs mid-day. It doesn’t mean all other power plants are turned off. Some power like nuclear or coal have thermal inertia, and so they aren’t worth trying to shut off for a few hours - it would cost them more to shut down and heat back up than to just pay the negative price to stay on. So this negative pricing just indicates that more solar is online than ever before, and the market is ripe for diversification of energy sources (or storage) to take advantage of price differentials. If other sources phase out, solar may meet the full summer demand for a few hours each day. Then eventually with batteries solar can meet the full demand for a few months straight. But it will never meet winter demands, especially with electrified heating.