There is wind at night. It's extremely unlikely that all renewables produce no power at all for a whole day. Batteries are perfectly fine to smooth over a 50% dip in power generation over a few hours. For longer periods (i.e. winter) you want power-to-gas and gas plants.

Also, 21 billion is actually not that much. Coal power costs about $2k per kW (or more[1]), so having enough coal plants to satisfy that demand would cost about 18 billion.

[1] https://www.synapse-energy.com/sites/default/files/SynapsePa...